1) If the Ring is sentient, as some suppose it to be, why doesn't it react at all when Gandalf recites the Ring incantation in the Black Speech at the Council of Elrond?
'Ash nazg durbatulûk, ash nazg gimbatul,ash nazg thrakatulûk agh burzum-ishi krimpatul.'The change in the wizard's voice was astounding. Suddenly it became menacing, powerful, harsh as stone. A shadow seemed to pass over the high sun, and the porch for a moment grew dark. All trembled, and the Elves stopped their ears.
Everything and everyone else has some reaction. Not the Ring.
2) If the Ring actually changes size, instead of just seeming to do so, might that not have something to do with Sauron's nature as a Maia who could change his size and appearance until his death in Númenor? Since Sauron put much of his power into the Ring, and since his ability to change his size appearance became severely limited thereafter, the Ring could well have an innate ability to adapt to the size of its possessor, which carried over from Sauron. This could also explain why the Ring does not change size when Bombadil handles it -- because he does not possess it.