eyes are bigger than one's stomach

(have) one's eyes are bigger than one's stomach
(have) one's eyes bigger than one's belly

The saying “your eyes are bigger than your stomach” is used to chastise people who appear to be taking more food than they can possibly eat. Also used to say that someone wants more than he or she can handle.
(по)жадничать; ||замахнуться на непомерное или непосильное; ≈ взяться за дело не по плечу; ≈ глаза завидущие — руки загребущие

Example 1: I can't finish this piece of cake. I'm afraid my eyes were bigger than my stomach as usual. (thefreedictionary.com)
Example 2: Try to take less food. Your eyes are bigger than your stomach at every meal. (thefreedictionary.com)
Example 3: He's thinking of acquiring a third business, but we think his eyes are bigger than his stomach. (dictionary.reference.com)

See also
[eyes bigger than one's stomach]