There was an article a little while back about kids getting fatter when they start school. I wish I could remember what it was about, but I just remember thinking that was totally me. In kindergarten I was normal sized (maybe a little larger) and then 1st grade hit and I ballooned. I wish I was at home cause I'd post the pictures. I tell people I looked like Bobby Hill (from the king of the hill cartoon) just with really long, permed hair when I was six.

I am not exaggerating. From then, my awkward phase lasted until I was about 17 and lost a little weight and started feeling better about myself.
My dad's girl friend brought out pictures from my brothers wedding when I was about 9 or 10, and its so embarassing and horrible. I think most kids are cute, but even my family laughs at how I looked then. It didn't help that I developed acne and man-boobs at 9. It was really torture being in my skin then, I think especially because no other kids my age understood.
For me, the school weigh-ins were bad, but I remember being tormented for things like.. physical fitness tests, or bringing "diet" food in for my lunch. Kids are horrible sometimes.
Anyway, I'm totally rambling but as a person who's never in my adult life been smaller than a size 16 (maybe for a day or two when I bought those skinny jeans) I thought I'd add my experiences.
I think for me, its more of an issue because weight and food and working out have been things I have thought about Every Single Day since I can remember. Thats, what, about 18 years at least? It's never been ok or comfortable in the sense that I was perfectly happy with myself, but obviously if I've been this way for so long I have been TOO comfortable to change it for any substantial amount of time.
I think people who are thin and gain weight don't have that looming over their heads the same way.