someone's eyes are bigger than their stomach
US /ˈsʌm.wʌnz aɪz ɑːr ˈbɪɡ.ər ðæn ðer ˈstʌm.ək/
UK /ˈsʌm.wʌnz aɪz ɑːr ˈbɪɡ.ər ðæn ðer ˈstʌm.ək/

1.
to take more food than one can actually eat
:
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He ordered three main courses, but his eyes were bigger than his stomach.
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I think my eyes were bigger than my stomach when I filled my plate at the buffet.