Where does that leave us former fat kids?
It's a truly remarkable feeling -- on one hand, you constantly feel good about yourself… on the other hand, you're always worried about reverting to how you used to look. This makes itself apparent in me by the way I feel after meals. I still have my "fat kid appetite" but always make sure that I don't go overboard during lunch.
Still, as a lover of junk food, it's hard to control my cravings. This leads to overreacting to minimal weight gain, followed by dieting, followed by minimal weight gain, etc., etc.
The feedback is simultaneously reassuring and cruel -- you want to not care what people think, but that's how former-fat-kid confidence is initially built out. It's up to you to learn how to filter the loop and trust yourself to remain confident even when you're stuffing yourself into a pair of jeans that makes you feel like a delicious, plump Jimmy Dean sausage.