They viewed satyrs as more "animalistic," which could account for their larger genitals. If the appeal of smaller genitals was that it made humans seem more civilized, intellectual, and less subject to their baser instincts, it makes sense for a satyr (not even fully human) to have gross, beastly genitals. Pan was a pal of Dionysus but he wasn't human. Source + memory of Greek mythology I learned in school.