From my experience...
1. older pictures
2. they've had "work" done
3. they have worked their butts off for their figure
4. older pictures
I dreaded my 10th and 20th reunions just b/c I am a shy person and was worried about how I'd be perceived...but I had fun, more at the 20th than the 10th.
I did go on a fairly extreme diet for the 20th...lost a bunch (gained it back, drat, now I'm exercising and it's going to be better for me). Several people at my reunion said that I looked JUST the same...that was nice to hear. Though they were also saying my hair was the same, and it was NOT the same, but that's OK, there's not a lot to do with curly hair if you don't want to straighten it.
About number 2....I'm sure many more people at the 20th had had work done than I knew. But two stood out. One woman had had so much work done that she was unrecognizable to all but those who knew her currently. She was wearing a deep v-neck dress with a halter...and NO bra...that made it pretty obvious she'd had at least ONE bit of work done...10 year reunion that could still be natural...20 year? Hmm.
The second...I didn't know she'd had any surgery. So I just hated her, just as I'd hated her since junior HS (long time girlfriend of one of my long-term crushes through school), because she still looked perfect. But then at one point, I found out later, DH and DS came looking for me (ticket was too expensive for DH to go as well, and kids weren't allowed, so they sat in the hotel room). Couldn't find me, and on the way back up in the elevator DH got to talking with a woman. She found out who he was, she knew me (shock!), and she went on about how much work she'd had done to look decent for the reunion. Oh what a lovely story....
Anyway...don't be weirded out if you're heavier than you think they are. If THEY were heavy, would you like them less? If no, then they probably won't like you less, just b/c you're carrying a bit extra. Go; you might have a good time! I loved reconnecting with some people, especially one old friend. Even though I was a wreck the months leading up to it, and I felt fat the whole time and worried my bra was showing (weird dress), I did have a really good time and I was glad I went.
Even though the cheerleaders/reunion coordinators still (still!) insisted on SCREAMING at us. Actually, that was funny, because one of them used a very naughty word in her pleas to get us to shush...when it was their sound system's fault..couldn't even hear their shrieks if you were 20+ feet from the speakers.