It seems there aren't many penises that are a perfect cylinder, but some are much more ovular than others. By this I mean the shape is relatively wider. Think of it this way: Imagine a perfet cylinder that was, say, 6" in circumference. Now stretch that out on the sides, so the circumference stays the same, but now it's just a wider oval. Wouldn't this cause more stretching in the vagina due to a now wider diameter?