the OUTSIDE of your leg hurt with the brooks? do you still have the brooks? I don't get how the outside of your leg has anything at all to do with the saddle.
There are a million threads on here about saddle issues, but you might just want to try lubrication?
I don't think this has anything to do with your weight; but poor fitting shorts could be an issue too, I guess. I'd be looking at saddle/bike fit first though. Good luck