I think I'll try and actually answer the posters original question of topic.
I'd imagine the reason we see so *few* people whom are overweight is, frankly, science.
We have seen "fat" people in Trek, though they are rare. We've certainly seen such as people age. Shatner in Generations. Chief O'Brien in DS9. Even Avery Brooks and Jonathan Frakes looked a little more rotund at different times. Scotty, of course, is likely the only real example of an "obese" primary actor/character.
I'd imagine that by the 24th century, there would be much easier means of keeping fit than there are today. Food would likely be less unhealthy. So we would have to work-out less to lose any extra. Certainly, in TNG, they all but outright said that replicators where programmed only for items of nutritional value, which had to have been code.
Lots of admirals and ambassadors and such have been on the rounder side.
Obesity isn't healthy. But combatting it isn't easy in our present times. Within say 100+ years, I'd imagine that a combination of things will change that. We may identify and be able to turn off obesity genes, we may be able to make healthier food which would in turn require less phsyical action to 'burn off' and so on.
We've never quite had a time of relative over-abundance like we do right now in human history. So of course we where bound to over-do it.
That said, there have always been fat folk and always will be. There where fat people in the dark ages. Granted, they tended to be rich, but... you get my point.
I think the real point here is that, right now, we're over-indulgent. We don't know how to stop, or we can't be bothered too, and so... we're here. You know? Hopefully it'll change. I know I could stand to lose a bit!
