I'm suggesting that the explanation "you don't see fat joggers because fat people don't jog" is as or more likely than the explanation "you don't see fat joggers because anyone who takes up jogging becomes thin so quickly that nobody sees them during the time when they are fat and jogging".

If fat people took up jogging and became thin, you would see fat joggers.

I think what he is saying is that your views are very black and white, and it's just not that simple. Maybe my definition of fat is different than yours, after working at a tech company for 5 years, but when I think fat I think 300lbs+, not 200lbs with a beer gut that can be jogged off in a few weeks. Some of these people have trouble walking to their cars without some kind of assistance, what is your solution for them? But hey, way to generalize a whole lot of people. Fat is fat, right?