I've been impressed when I read that someone tries to lose x number of pounds and they increase their activity level. And increase it more. And increase it more. Until finally some weight starts to fall off.... reluctantly, but it does go. And not even that much!
Plenty of active people are fat. Plenty of vegetarians are fat. It's not a predictable process, losing weight with activity, especially when you can lose pounds simply by finding the right diet. However, being active has a lot of nice side effects, which of course vary from person to person. Not unlike dieting.