Interesting approach, but "unstretched" by anything penetrating a real dog vagina ? would be quite low in variability of diameter.
It's when something actually penetrates the composition of different tissue layers, that it feels differently due to muscle tissue, tendons axially and less radially (aside those linked to hind leg structures), fat tissue which is very soft..
As it would be too much effort for private production runs to actually use different shore hardness silicone to flood different areas, I assume your "different diameter" approach is simulating this by varying back pressure through elasticity stretched more or less?