I agree that you can't tell too much about someone's lifestyle from their appearance. Apart from some obvious superficial things, like that a really large person probably isn't an elite gymnast and a really slender person probably hasn't spent the last summer at sumo camp.

For example, it's not helpful to look at a thinnish person and assume they're healthy. You can get thin people who barely do any exercise, don't eat very well or often, are quite unfit and those around them insist there's no issue. And then there are wonderfully fit and energetic people who are heavier than they need to be.

I guess the fat and skinny thing is just a cultural values thing, as well as a language thing - call someone slender and it doesn't seem so bad. I think it's wise to think before pretty much every comment, I know of short and tall people who get inane or thoughtless comments too.