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!
See also: belly, big, eye
Farlex Dictionary of Idioms.

someone's eyes are bigger than their belly

or

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.
See also: belly, big, eye
Collins COBUILD Idioms Dictionary, 3rd ed.
See also:
  • (one's) eyes are bigger than (one's) belly
  • belies
  • cooking for one
  • as one door closes, another (one) opens
  • as one door closes, another one opens
  • as one door closes, another opens
  • drive (one) out of office
  • force (one) out of office
  • force out of office
  • give (one) (one's) head