Hm, who honestly COULD eat more? This takes a lot of thinking, as there are many-a-factors to find the answer. It's more than just "Who can swallow more", since they each have an infinite cap
One the one hand, there's Yoshi. Long tongue to get the food, abilty to quickly digest food into eggs. Also, it must have taste buds, since they all have prefered fruits to eat.
Then there is Kirby. Powerful lungs, which I would consider a part of his digestive system, large mouth to accommodate any size enemy.
My conclusion? Yoshi could eat more. While Kirby definately is a contender, he can only swallow one 'powered' enemy before he becomes full. Then he has to get rid of it before he can eat again. Yoshi can continuously eat and eat, quickly working it through its system. Interesting note, though, while Yoshi can eat more, Kirby is usually the only one to eat for the traditional sense, as in gaining nutrition. He eats to restore health in all his games. Yoshi did so in Yoshi's Island, and kinda did in SMS, but that was only to restore juice.
Oh, and I.S., Yoshis tounge would reach for Kirby, though since Kirby would be inhaling, Yoshi would be pulled closer. Yoshi, if he was thinking, would aim for the center, which would be the back of the mouth of Kiby. As they began to eat, they would try to open their mouths as wide as possible, always trying to be more than the other, until Yoshis tounge pulled him to Kirby, and Kirby's suction pulling Yoshi, they would meet, and, essentially meet in a fighting kiss. No one would be able to get their mouth completely around the other so long as the other was still trying. Nasty, kinda disturbing, and a stalemate.