For standard bred birds, the Plymouth Rocks SOP wt is 9.5 lbs for a cock and 7.5 lbs for a hen. Rhode Island Reds SOP wt is 8.5 lbs for a cock and 6.5 lbs for a hen.
There will be variations of course but can't really go by their weights as a determining factor of what breed they are - especially since RIRs should be smaller than PRs.
Sometimes it is really hard to ID breeds when you have hatchery birds, because they have lost some of their "type" and when you get birds of similar colors or "mutts", without the best body type, it's a shot in the dark to ID the breed if you didn't get them identified when you bought them.
When I first started getting into chickens as an adult, I didn't realize that there was so many subtle things that make up a chicken besides the obvious stuff. Once I saw some old - line, standard bred chickens compared next to hatchery birds - WOW. The difference was amazing to me. But before, all I had seen was what most people see - pretty feathers. To help me with my birds, learning to better identify those other more subtle traits, I turn photos of our chickens to black and white. That way I am not distracted by feather color.