I think they are completely different things. The penis and vagina are the main elements for sex and reproduction. Whether a girl has small or big boobs is only extra. However, when it comes down to sex for it to be fun, neither the guy or the woman wants to doing it with a small penis. Sex, although, can still be fun whether the girl has big or small breasts. Think about it this way, how many girls would you still want to have sex with even though they have small boobs. And then think about asking girls how many guys would they want to have sex with who have small penis', I think the results would be very one sided.