Definitely regional, there's a preference for bigger dicks well before the romans, too.

I dunno when "bigger dick = hornier" came about, but I know it was at least Greek. But then romans valued reserve, things in proportion. Sometimes they let loose but they felt there was a time and place, and a man who couldn't control himself wouldn't be favored, and a big dick meant exactly that. Impulse control, not a specific size preference.

Its very interesting, honestly.