надуться как мышь на крупу (nadut'sya kak mysh' na krupu)

Literally: To puff up like a mouse on groats

Example: “Из-за отменённой поездки он надулся как мышь на крупу.” — He sulked like a child over the canceled trip.

Origin: To puff up "like a mouse at the groats" is to sulk and take offense. Michelson's phrase dictionary records it with a quotation from Pisemsky's Leshy (1850s), where a manager "puffed up like a mouse at the groats" and barely nodded. Why the mouse is offended is not explained; the comic picture of a tiny animal swelling with indignation is the point.