my2geldings is right about the term overweight being different for every person and their body type. I've known people who weigh less than me look heavier than me and people who weigh more than me look much lighter than me. I think the more important thing is how healthy you are physically, such as cardio, joints, etc. and your riding ability.
When I was a teenager, I was the oldest teen at my barn at 15. I hated going to any clinics/shows being offered because, while I was not very overweight, I was still the heaviest rider at the barn compared to the younger teenage girls. And most of them let me know. So it can definitely be hard in a group/boarding barn environment if you are concerned of what others will think about you in that way. But my riding ability was great and getting better all the time with lessons and my instructor said the lesson horses I was riding would not have a problem. So I think at the time my weight problem was more in my head than actually bothering any of the horses I was riding. And as with all social things, don't let what other people may say to you get to you. I didn't and stuck around that barn for another six years and learned a ton.
But again, there are still several factors to consider when thinking about how it affects your horse. If you are still concerned I would say maybe check with your vet, or possibly someone specializing in equine massage or chiropractic work, but I'm sure they would probably tell you what most of us are telling you. And as far as trying to lose weight, just do what makes you feel good. At the level that most of us ride we don't have to be star athletes. Just do what makes sense to you and what is best for you and your horse.