Actually that ^^^^^^ reply could find you with a "hostile workplace" action against you in a lot of workplaces. And thinking it through, rightly so. I know a lot of "fat" people who have every right to be the weight they are. I may internally pity them for it, but it creates more heat than light.
Might be more appropriate to reply with something like "Yeah, I'm too cheap to keep fit by going to the gym." Allows the other party to come to their own conclusion (or just confuse them further....).