Speaking of Yoshi, I am on a SMW kick right now, and have noted a couple of things about Yoshi in this game..
Yoshi can eat virtually every enemy in the game with little/no problem, right? Well, how is it that he can't swallow Koopa shells (regular and Masked-Kooopa) but he can swallow enemies that have fire-proof shells (Buzzy Beetles, Spinies, Spiketops) with no problem at all, even though they are shelled creatures?
My theory is that there is something about the fireproofness that makes them easier for Yoshi to digest.