(one's) eyes are bigger than (one's) stomach
1. One has taken more food than one is actually capable of eating My eyes are always bigger than my stomach when I go to all-you-can-eat buffets! Can't finish your meal, huh? I guess your eyes were bigger than your stomach!
2. One has taken on more than one can reasonably handle. Your eyes are bigger than your stomach if you think you can get all this work done by the end of the week.
eyes are bigger than one's stomach, one's
Someone wants more than he or she can handle, as in He's thinking of acquiring a third business, but we think his eyes are bigger than his stomach . This expression alludes to someone taking more food than he or she can digest. [Late 1500s]