It' all relative when it comes to women's perceptions of penis size. If her first boyfriend had a 3 inch hard prick and then her next one has a 5 inch long dick, then she think it's a "big penis". Likewise, if her first boyfriend had a thick 8 inch long schlong and the next guy has a 7 incher (which is really scientifically within the 90th percentile for size), she won't think it's a "big penis". It all depends on their previous experience.
Whoa! *girls thinks to herself how much bigger his penis is compared to her previous boyfriends*, that's a damn big penis! Must be a foot long!