Labia majora are the two outer layers of the vulva, which together with the labia minora form the labia. Labia majora is most often shown covering the labia minora, vaginal opening and urethra in anime/hentai art, as seen in the tag cleft of venus. Such posts should not be tagged with long labia, unless both variations are shown.
It can be long labia even if it's naturally small but it's being stretched.
The medical term is labial hypertrophy. It's a harmless variation from female to female and it is quite common. The main deciding factor is genes, but puberty, chronic hormonal imbalance, and pregnancy can enlarge the labia oftentimes permanently.
