someone's eyes are bigger than their belly
(one's) eyes are bigger than (one's) belly
One often takes or has taken more food than one is actually capable of eating. My eyes are always bigger than my belly when I go to all-you-can-eat buffets! Can't finish your meal, huh? I guess your eyes were bigger than your belly!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
someone's eyes are bigger than their bellyor
someone's eyes are bigger than their stomach
COMMON If you say that someone's eyes are bigger than their belly or their stomach, you mean that they thought they could eat more food than they really could. Sorry, I can't finish this — my eyes were bigger than my belly. Note: You can also say that someone has eyes bigger than their belly or their stomach. Dan always did have eyes bigger than his belly.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012