I don't find that hard to believe at all! I'd say that the "overweight" ratio would probably be pretty close to normal, maybe slightly less...but obesity really only occurs when you start adding effed up hormones and way high levels of saturated fat to your diet.
I way about the same I did when I was omni - 145 lbs., 5'3", which I'd say is about 5-10 pounds overweight. It's actually a VERY consistent number, only changes when I fast (I used to be anorexic) or exercise (when I was in crew [rowing], I got down to 135 lbs., including TONS of muscle weight - I went from not being able to run a full mile to being able to run six miles in about three months). So I have a feeling that if I exercised regularly, I'd be a healthy weight. When I was omni, though, even when I exercised I was still overweight. And my weight fluctuated a LOT more back then too. If I ate fast food three days in a row, I'd gain like three or four pounds. Now I can eat a lot more junk (and I do), but I don't really gain weight. Most of the time I eat healthily, and I don't lose much weight either.
Part of this could be that my metabolism is shot from when I had an eating disorder. *TRIGGER* - please don't read this if you're struggling with body image. I was alternating anorexic/bulimic/binger for about five years. My lowest weight was about 115, which is when my boyfriend starting complaining about my ribs poking him in bed. >< That was due to not eating for two or three days in a row. *END TRIGGER* I've only been pescetarian for about a year and a half, and only vegan for about two and a half months, so I don't expect my weight to even out for a while. But I know I'd be in way worse shape, health-wise and eating-disorder wise (due to the addictiveness of dairy and fast food) were I not vegan.
ETA - Sorry I got off-topic!