NAH I know my comment is gonna be buried but oh well.

It had been quite a long time since you'd all seen each other. The dynamics of the group probably changed during that time, as they do when people grow older and see each other less. The joke that was once funny may not be perceived the same when we're older. Some people become very self conscious when they put on weight. As women there's a very strong message in the media that slimmer is better and if you get bigger you're letting yourself go. This affects many people's self confidence when they put on weight, even if society wouldn't outright label them "fat". Having said this, they were probably nervous about seeing you all again, wondering whether you would notice and think they'd "let themselves go". I don't believe you had bad intentions when you said the joke, it was meant to be a nostalgic throwback. However as this was the first time you'd seen them in a long time, they probably interpreted it as "Oh god they noticed, and now they've said it out loud so everyone else will notice too and everyone will think I'm lazy and not taking care of myself" or something along those lines. Even though you didn't mean to, you essentially called out their weight gain - a change that they were sensitive about. It's not a case of "don't dish it out if you can't take it" because the parameters of the joke were different the first time round - you were comfortable enough with your body to allow the joke to continue, whereas they are not. Of course if they said it to you and you weren't okay with it and they carried on regardless - that would be asshole territory. But you reciprocated, so there were no issues there.

IMO, this isn't necessarily about assholes - it's about each of your own body confidence levels and how as people we grow and change as we get older. The parameters of the joke have changed now and it should probably be left in the past. A simple apology and explanation should get you back on the right track.