your eyes are bigger than your stomach
(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.