Why do people say "bigger is better" in terms of size?

As a girl, I don't get this at all. In my experiences, bigger = more pain. On average, an un is only about 3 - 4 inches deep. When , it can handle a little more inches but not much. So, imagine only being able to handle 6 inches and getting 8 inside you. Geez, we have organs inside too! So, why do people say such things?