Why surprise?  It will be explained by reference to the skull's weight (in the second verse) and then, as the description extends, to a wider sense of its fragility, a sense of how such a small thing can contain so much of literally vital importance.  The little surprise will broaden into something like an epiphany.  Balance is a nice verb, suggesting not only a literal physical weighing, but also careful consideration.  And some measure of physical care too - this verb, taken with the adjective small, attached to the skull from the outset, suggests what will be developed by the poem: a quality of tenderness applied to this object - which as the poem goes on, will be applied to the speaker's own sense of herself.

Like any poem setting out to describe any object, it begins with a simile, a comparison - and, as with any poem, the simile must first be surprising, and then seen to be appropriate.  The point is that at this stage in the poem, this skull is taken as just any object - the poem will only gradually engage with it.  Here, the comparison is deliberately almost playful, innocuous, not yet suggesting all that will be read into the skull.  It is compared to a musical instrument: .

an ocarina; a small wind instrument.  The holes, the weight, the fact that it fits in the hand, all provoke the comparison.
But the poem can begin to turn on that comparison.  As you would with a wind instrument, she blows into it - but 'breath' is a figure for Life itself.  Blowing into the ocarina/skull becomes more like giving it the kiss of life - which obviously must fail - it's much too late!

Cry,holds my breath both begin the transition from a mere object into something with a sort of life of its own.  Cry especially replaces what might have been more expected if you were talking about its closeness to a musical instrument - rather than refer to its note or sound, the poem uses a word with associations of human pain, and also, perhaps, with the cry of a new-born infant.  The skull can hold breath again - but only ironically (perhaps suggested by the reference to its grin).  It can only be given back a kind of temporary semblance of life - only as long as I exhale.  Compared to an ocarina, its note is short (after all a skull was not designed to be a musical instrument!) - but the phrase which conveys that is far richer than if that were all that was at issue:

The phrase could be applied to sum up a human life, viewed in a particular way: a transient sadness. Already the poem has moved from considering a mere object to what it might suggest about the largest things.

However, the poem is intent on not just jumping into these large issues.  If a vanishing sigh is a rich, resonant phrase, the first line of the next verse pulls the poem back into something more banal and ordinary; we're back to the ordinary occasion of the poem:

But note that what the phrase would normally suggest - with my head in my hands - would be something like despair, especially when followed by the adjective appalled.  But it is here more literally true: the skull she owns as an ornament is held in her hands.  But something of the despair of the normal suggestion is still conveyed, still used by the poem.  Why appalled ?  Because it is much lighter than she might have expected.  And why expect it to be heavier?  Because your real skull contains everything important about you: your memories, your desires, your whole sense of yourself.  The lightness of a real skull makes all those things seem much flimsier than you might hope.  Notice how the poem only gradually moves the reactions forward: it has gone from the (fairly neutral) with some surprise of the first line, through mildly alarmed to now appalled.  The fourth line will bring disturbing.  
The light weight of the skull is compared to a deck of cards (just as a readily available comparison - or is there some suggestion of fate here?), then to the weight of a slim volume of verse.  This second comparison does seem more obviously significant: it perhaps feels to the speaker that that is only what her skull, her mind, has managed to produce in her life.  
But there is something more - or should be something more:

This more than simply physical quality is what is disturbing.  But the poem, having reached the edge of a large, important issue: are we more than our simple physical being, swerves away  - or rather, the speaker's reactions in the poem swerve away.

The swerve away from the large, ultimate, abstract question takes a double form: into a kind of sentimental identification (the kiss), then into a kind of joking about (treating the skull like a ventriloquist's dummy, with the speaker as a very bad ventriloquist, something like Tommy Cooper).  But these two reactions are recorded in the poem in the form of a question: the speaker's reactions are a mystery to herself: why react in this way?  What is being avoided by these reactions?  Perhaps the implications of the skull's weight - all that the second verse seemed to be approaching.

The third verse settles on a kind of practical compromise.  Rather maybe than think about the skull, do something practical with it - clean it up.  But even the activity brings with it words with more serious associations : dust (the word bringing with it perhaps associations with the burial service) [even the comparison of the dust to sand from a swimming cap brings with it some link to lost time: in the volume,  a poem like Beachcomber associates swimming at the seaside with the lost memories of childhood], and drying the skull is associated with drying a newborn baby.  So we have the whole of a life implicit in the skull.
 
The practical care of the skull in fact moves the emotional attitude to the skull forward.  The verse moves from the impersonal dust to an intimate association of it with a firstborn baby.  In the last sentence of the verse the identification with the skull has become so close that what is read in the feel of it are the speaker's own memories - as though the scar of the speaker's own accident can be felt on the surface of the skull:

Perhaps behind these lines might be a reference to a 'real' accident.  That would be a purely private allusion, which would leave the lines simply enigmatic to us ordinary readers.  (Many modern poems do in fact do that - perhaps maddeningly so?)  But there's maybe something else we could say about these particular lines.  The notion of descent, a 'fall' into birth (and it was with birth that the last sentence left us with) is familiar from Neoplatonic philosophy: an influence on some parts of Christianity, but also in some Romantic English poems (for example, it's active in Wordsworth's Intimations of Immortality). The word for in falling for sheer love then perhaps is worth some consideration.  it obviously shows that a reason is being given for that fall, but it could be just a practical, immediate reason for an accident (something like: I was so upset by an unhappy love affair that I fell down some stairs and gashed my head), or it could suggest a more abstract, larger motive tied to the notion of Neoplatonic fall.  Then it would go something like: in order to feel human love - with its neediness, its sharp intensities (neither of which would be felt in a Neoplatonic Heaven), I descended into the flesh - ie I agreed to become human, be born, with all the limitations, the unhappinesses, of that state.  Hence the 'treacherousness' of the stairs - betrayed by a promise of feelings.  (Sheer love also then becomes a nice rich phrase: sheer suggesting pure love, for love alone - but also tied to the idea of a sheer drop: an absolute and sudden descent, with no stages in-between.)

It's a concern with the idea of Love which opens the last verse

If love has been the 'motive' for a descent into limited human life, then the attitude of the poem, realising this, is sceptical.  Note how the adjective hollow has become transferred from the skull to the nouns.  (What would the hollow nouns be apart from Love?  Words like Beauty, Truth, Justice, perhaps - all those large abstractions, which we use to label those values, ideals, principles - which fill our minds as aims and desires while we are alive.)  What attitude can be felt towards Life once this reductive scepticism is felt?  Well, the poem entertains several different attitudes briefly.  Firstly, you can feel pity for others and for yourself:

or, alternatively,

-ie, a kind of sardonic black humour at having seen through how Life cheats (of course, it has been usual to refer to the grin on skulls - as the poem itself did earlier - precisely because of this: they have seen through Life).  Finally, there is possible just simple identification with those others who have been cheated by Life:

It is with this identification that the poem finishes on with its last sentence: