Not a lady, but after 30yrs of marriage, my wife just admitted to me that she has only cum from just penetration alone about 3-4 times in her life. (Counting all 7 bf's before me / and I knew it wasn't me) She didn’t want to tell me who it was with, but I convinced her I wouldn’t be mad or upset. She said it was guy she dated and fucked at a sports camp years ago (long before us) and that he was very thick and she came every time from just his cock. Us guys always think of length, but thickness is what seems to be the key. It makes sense to me, since the clitoral legs run down the vaginal walls and the G-spot needs pressure, that a thicker cock would stimulate her more. Also I believe from what my wife has told me that the visual of a big thick one is a turn on for many women.