Big Cock is functionally the same body-trait tag as 'big penis,' just using the more colloquial term that shows up in casual search queries and on some of the source sites Top Hentai aggregates from. Manga and doujinshi filed under this tag feature a male or futanari character with above-average anatomy, often central to the scene's visual emphasis. If 'big penis' doesn't turn up the title you remember, searching big cock hentai on Top Hentai covers the same ground under the alternate name.
