How heavy a rider can a horse carry?

also know as "horse size vs. rider size part 2". And I'm fearing I may need my flame retardant over-coat for this one :-( Anyway, I used to ride, I loved to ride, and then I gained a lot of weight. A lot. Now I'm scared to ride for fear I'll cause undo stress on the horse. I really want to ride again - *sigh* - maybe that will help me diet. But is there some sort of ratio so I can figure how big a horse would be needed to carry me at my current weight, and/or how much I have to lose to ride the horses we own now? I don't want to squash anyone flat ;-)