light gathering comparison?

Can someone explain to me how different types of scopes in different sizes compare as far as light gathering? What I mean is, it seems like lots of people use high end refractors that are often only 90mm or so for deep space viewing/photography. I'm just getting started, but I got a 6" reflector and can't imagine that type of viewing with anything smaller. As a matter of fact, I've been thinking I should have gone for a 10". Now I know I'm comparing apples to oranges here, but is there some kind of rough comparison that can be made, like 6" reflector = x" refractor? Thanks, Bill