I don't think weight is the main factor - saddle fit is. In most english saddles, you should be able to fit a palm behind your butt on the saddle. This doesn't go for western saddles, because the seat in these are different, but generally for all saddles, if you'r bum goes over the cantle, if you can just barely squeeze your butt into the saddle, or if your butt only fits into a saddle that is too long for your horses back - then you're too big, no discussions. If you're bodyweight gets too far back in the saddle, you're putting the wrong pressure on the horses back.
Sounds simple, yet many ignore it(or just never heard/thought about it).
Rider weight and the condition, age and build of the horse, still matters of course. But i think saddle fit for the rider, is the best guideline :)
So if it fits, you sits! ;) I think it's great that you care and think about it though, i think we all should.