Well, I'm not professing to be an expert on the matter, but...
I've noticed that the bubbles in my beer get finer after a bit of aging/conditioning. This leads me to think that it has something to do with the size of the particles that are acting as nucleation sites - the larger particles will precipitate to the bottom after a while, only leaving smaller nucleation sites.
This may also be supported by my experience using different ingredients. Some give a dense creamy head (small bubbles) and others a more fluffy, short lived head (large bubbles). Again, my theory is that different ingredients leave different sized particles in the beer, thus different bubble sizes.
Of course I'd be gladly corrected if there's something else going on. Actually, come to think of it, seeing as the foam is generally made from proteins,maybe it has more to do with the types of proteins in the beer?
Sorry for the ramble, typing while thinking.