It gains body skills and fitness interest, but not actually slim sims down. Am I the only one to have this issue?
Not sure, now that you mention it. I know I had a sim-6 who gained body skills by jumping rope, but can't recall if he gained any slimming effect.
On the other hand, it reminds me that there was one activity I saw the other day where the sims appeared to slim down well enough, but gained no body skill at all (dancing?). I'll have to check for that now too.