Kirby: He's eaten enemies 50X his own size before (e.i Bonkers, even some VERY large bosses)
Yoshi: It depends on which Yoshi you're talking about. Baby Yoshi in PM: TTYD could eat just about every baddy out there, except for big bosses (e.i the Dragon sibilings, Shadown queen) The bigger Yoshi's sometime don't even bother eating bigger baddies.
I think Kirby would be able to eat more. Although I think we're all forgetting someone who could out eat the both of them........a certain guy who's fatter than Mario and wears a yellow shirt and purple slacks.