Hey Mutt, I think part of the problem is that the gals with the big racks in retro porn (for instance Christy Canyon) would be considered "fat" today.
Standards have changed and for some reason bony bitches with tatoos on their backs are considered "hot" while a girl who's bigger than a size 6 doesn't even think about doing porn because she thinks she's too fat.....
The boobs are still out there, they're just not being recruited or accepted by the casting agents.
