OMG that's so true. In my experience, all the pretty thin girls in HS are now married and out-of-shape...all the ones who like me were never thin are in general leading much healthier lifestyles. YMMV. No need to hide.
If it helps, I may be fat but I've gotten several personal messages from girls I went to HS with, some of whom I wasn't even friends with, asking me what my secret is for looking so young. I'm 39 but I get guessed as being between 30-35 (no wrinkles). It's genetics, but I always say, no kids!

So really don't be afraid to join and don't be afraid to put a realistic (but flattering of course) photo up because...
NOBODY LOOKS LIKE THEY DID IN HIGH SCHOOL ANYMORE!!!! I promise!!!! My ex-husband was absolutely the most handsome boy in our high school class, tall, thin, blonde (he won the "Best Looking" in our yearbook) ~ now he looks like Homer Simpson minus the beer gut.