That's an awfully complex question.
It can, but that doesn't mean that it will, so the answer is kind of yes-and-no. It depends on how you ride, how long you ride, and the horse you ride. For instance, riding a horse that has back pain will obviously hurt. However, what about riding a healthy horse? I think that if someone can ride well (like not bouncing around a lot, keeping balance well, and keeping cues to a minimum), and doesn't ride for a long time, then it shouldn't hurt. About the weight rule you mentioned, I heard that a rider and their tack should never exceed 15% of their horses weight. I believe that is true, but it's more of a general guideline than a hard and fast rule. If someone rides often, they should probably observe it more strictly than someone who only rides every couple of weeks. Another factor to consider is what you do while you ride- are you doing something physically strenuous, (like jumping high or jumping really often, or contesting) that can affect the wear of the horses joints over time? So, it depends on a lot of factors. I think the best judge of weather to ride or not is the horse- does he/she seem happy and comfortable while riding, or do they seem uncomfortable, subdued, or in obvious pain? But even then, horses have a tendency to hide pain. (Think about their prey psychology- the horse that seems hurt will be selected by hunters first.) And then there's saddle fit, which I won't get into in this post because it in and of itself is complex enough that entire books could (and have been) devoted to it. When I said that this is a complex question, it is a VERY complex question (and there are probably additional factor that I'm not thinking of right now.)
Sorry if the earlier paragraph got you worried. That wasn't the goal of that paragraph- the goal was to give you some examples of things to consider. If you care, I personally have ridden my horse less than 10 times since last summer, but that's mainly because she has lameness issues (I only ride when she hasn't been lame within the last week or two). I really don't know how much I would ride if she didn't. I guess that if you're considering riding again, think about if your horse is physically capable first, and if your saddle fits alright. Riding a few times for about 15-20 minutes each time won't hurt your horse, since most damage from riding is built up over time. You can try out riding again if you want, and see if both of you are comfortable with it. But I wouldn't worry too much about the weight, especially if at first you're only riding for a short time anyway. Collection does help the horse carry the weight, but remember that it your horse hasn't done it in a while, it is pretty tiring for them. Anyway, good luck with whatever you decide to do! And don't feel nervous about posting any questions you have. How is anyone supposed to learn or get smarter if they don't use their brain once in a while?