For what it's worth, Japanese erotic art, or shunga, typically exaggerated both genitals and not just the penis.
Beyond artistic efforts to draw the viewer to "the action," I'm not really sure why they did that, though. I study Japanese history and can tell you that, during WWII, when an American prisoner was captured, it was very likely for them to be castrated by Japanese troops. They did some seriously fucked up things with the genitals of American troops, but there are pictures of triumphant Japanese soldiers that are nude and have full erections as they stand by their collection of severed Americans penises.