I think derefr might be talking about the cells that form the walls of the capillaries being bigger, so you can't really fit them in the places you need them, a

I think derefr might be talking about the cells that form the walls of the capillaries being bigger, so you can't really fit them in the places you need them, and if you tried they'd be too narrow. Except replace "you" with evolution and delete "tried".