Your eyes are bigger than your stomach

DEFINITION: an expression used when somebody wants more food than he or she can eat, or when someone takes more food than he or she could possibly finish.

NOTE: During the holidays, there is typically an abundance of food at parties and celebrations where people can easily over-indulge (which is why many lament they have to eat healthier in January after the holidays are over…)

Examples:

“Wow, that’s a lot on your plate—I think your eyes are bigger than your stomach!”
“I always take more than I can finish eating when I go to an all-you-can-eat buffet. My eyes are bigger than my stomach!”