So, I work in a fairly laid-back environment; my college's bookstore. I get along pretty well with all of my co-workers, from the sweet middle-aged ladies, right down to the lude stock guys who make the occasional mildly inappropriate joke.
One of the guys I work with... is probably in his early 30s. He's one of those guys... thinks he's pretty macho. Makes a lot of stupid jokes which I roll my eyes at, and retort with snappy sarcastic comments. But we get along fine, crack jokes with each other, whatever. Really off-hand random stuff, like him coming back behind the counter and me shooting him a fake glare and saying, "Who said you could be back here?" or me tidying something up and him snapping, "Don't do that," for no apparent reason other than to give me the opportunity to tell him to "stuff it."
So anyhow, he comes in the other evening, and he goes, "So I was looking at your Facebook photos."
"Yeah?" (That's fine, I understand that accepting someone as a friend on Facebook allows them access to your photos. I can't fault someone for creeping, it's part of the deal you're making when you "confirm friend.")
"Yeah. I mean, I know you mentioned that you used to weigh a bit more, but OH MY GOD. I mean, you look like a totally different person. Like, I wouldn't have known it was you if you hadn't been tagged!"
Okay. I mean, not exactly overly tactful, but he's a guy. He clearly intended it as a compliment, so I half-laughed and said, "Ah.. yeah, thanks."
However, over the course of that shift, he brought it up again about 7 more times. And I overheard him telling one of the guys in the stock room who I'd had a crush on for ages about how much more I used to weigh. (*facepalm*)
Still, I just... shook it off, ranted momentarily to one of my female coworkers about how it was pretty tactless, then dropped it.
So, my last shift at work, same guy was in. One of my coworkers had brought in Lindt chocolate truffle things and offered me one. I decided to have one. I was unwrapping it when I heard my name. I looked up, and he was looking at me from his post by the door where he was playing guard dog to the onslaught of beginning-of-semester students.
"Jessicaaaa..."
"What?"
"Don't eeeat thaaat."
At first I thought he was just being stupid, like the standard joke-y, "Don't do that," crap we sometimes pull for no other reason than to be contrary. So I snapped back, "Psssh, I'll eat what I want."
And he replies with a half-shrug: "...just remember."
And suddenly, I realize he's referring to my weight. He was telling me not to eat a truffle because I might get FAT again! Honestly, if I'd had a baseball bat, he would have been in serious danger at that moment. I was just shocked that he could be so incredibly tactless!

Otherwise since it's a co-worker you could threaten him with a harassment complaint for repeated comments about your body.
Too funny, definitely a great comeback!
Try to ignore his weight comments and hopefully he'll get the idea, if not then it might be time for a serious smack-down.
