I'm a skinny runt and always trying to get my overweight friend out on his bike which is like taking blood from a stone. If I did manage to get him out and someone said a comment like that I'm not sure what I would do, I mean I'm not sure what I wouldn't do.
Like a fair few people, its not the size of the person, its the bike! "oh what bike/forks have they got" and a smile & a wave if they catch me checking the bike out.
The only time I have a little voice in my head is if the ass crack is showing, again I wouldn't say a thing, at least they are trying to do something about it.
I wouldn't say a word to anyone, fat or not. Now if they were a close friend, then I'd rip it to them as I'd know I'd get it back (banter).