eat like a bird
Eat very little, as in Jan is very thinshe eats like a bird. This simile alludes to the mistaken impression that birds don't eat much (they actually do, relative to their size), and dates from the first half of the 1900s. An antonym is eat like a horse, dating from the early 1700s, and alluding to the tendency of horses to eat whatever food is available. For example, I never have enough food for Ellenshe eats like a horse!