Ok, my druid is hungry and nealry out of supplies in an isolated area of the dungeon. She changes into a small(er) animal say a typical housecat, would a single rat (if eaten in animal form) fill her up (and when she changes back would her hunger be sated? A small (or smaller) sized creature eats and drinks less but does those eeffects "carry over"?
Err under 3.X your character will not die because of lack of food.
As to your rule question, there appears to be no carry over from eating while small and returning to medium. Not good for your character right now, however on the plus side becoming large does not appear to require eating more food.
"Small beings can have small wisdom," the dragon said. "And small wise beings are better than small fools. Listen: Wisdom is caring for afterwards." "Caring for afterwards ...? Ker repeated this without understanding. "After action, afterwards," the dragon said. "Choose the afterwards first, then the action. Fools choose action first." "Judgement" copyright 2003 by Elizabeth Moon