I only play Prosperity style (even in TS3). That said, I ended up with a random sim who was athletic among other things. When I started her, she was normal weight - midway between fat and skinny. I noticed after playing her a while that she was getting fat. I figured it was baby-weight as she'd just given birth (I can attest to that, for sure!). So I put her on the treadmill and got her back down to a nice slim shape. Then I went on about taking care of the other sims in the household. Next thing I know, I look at her and she's fat again!
Back to the treadmill. Skinnied her up again then started to watch her like a hawk to see if she was eating when she wasn't hungry or something similar. What I saw made my laugh so hard my husband asked me what was so funny (and he's used to me snickering at my sims antics). When she wasn't busy doing something else she was eating. And it's not just eating...she was eating leftover birthday cake. And gaining weight like crazy! So, after my laughing fit, I resigned myself to just keeping an eye on her and putting her on the treadmill after a cake binge. I swear she gained weight faster than any sim I have ever seen before! Even eating just to fill her hunger bar made her get fat. Then, her daughter evidently got the fat 'gene' from her and does the same thing. No matter what I do, she's a fatty. I've finally given up on hoping they'll ever be anything but BBW...heaven knows there's enough of us in the real world, that I know sometimes you just can't fight fate.