Oh, I’ve seen the difference when I have lost weight.
Some of it is definitely confidence, most certainly.
I learned (and saw) the most dramatic difference some years ago when I went from a size 24 to a size 14 at less than a year.
The thing was, I didn’t know that size 14 was not that thin. It was the size that most of my family members were at, and I thought they all looked fine. (And yes, bone structure does have some play in this–my mom and sisters don’t look that fat at size 12-14 and no one else seems to think they are fat, so are they fat? Well, it depends on who you ask, I guess.)
Anyway, the first thing I learned was that some of my friends didn’t take my weight loss all that graciously. They resented me getting a new wardrobe (I had been waiting a very long time for that wardrobe, dammit!). One girl especially didn’t like that I actually was a size smaller than her. (She’d enjoyed me “envying” her when I was a size 24, apparently, and didn’t want to let that go.) She was very angry one time when a lot of us were on a trip and I offered to loan her some jeans because she’s torn her last good pair. Very offended, she was. I hadn’t anticipated that someone who had pretended to be “encouraging” of my weight loss actually resented it. Fortunately, my friends got over their pettiness in time.
Another thing I learned was that my friends (who’d I always envied at their size 12-14-16) were more miserable than I was. I was a size 14. I was elated. They were miserable. I thought I looked great at size 14. They were not happy with the they looked (apparently). To me, not having to huff and puff when going up a hill was a huge deal and I was incredibly thrilled. The hell with how I looked–I could walk up a damned hill!
So it was a really good learning experience. I saw that I could be fatter than some people, and yet happier than they were. And I saw that some of them were really pissed off by this. How dare I be happy? Couldn’t I still be miserable? Couldn’t I torment myself until I was a size 8? Couldn’t I envy the ones who were a size 8 and whine that I wanted to look like them? What the hell was wrong with me, being happy at size 14?
Now I’ve gained some of the weight back. It sucks (but I’m starting to exercise and I’m crossing my fingers). But even though my weight is back up (not back up to a size 24, but damn–it’s up), my confidence has not waned as much. I refuse to be that same miserable person I was at size 24 all that while ago. It’s just not worth it.
And I find that what shrew says also has some bearing in my own life. Gaining weight is a great way to put a stop to the leers and gropes. I think that’s what made me (unconciously) gain weight at about age 15.