Because even the well endowed can be bottom bitches.
When a larger than average penis is locked away in chastity.
Typically shown through a larger than average or tight chastity device, or with before and after imagery.
This tag exists to more easily differentiate images where a well endowed character wears a chastity device, from images where a character has a chastity device and a different character is well endowed.
It should generally be applied if a post with a chastity_device tag has the big_penis tag, and it is referring to the character wearing the chastity device rather than another character in the picture. Relevant tags:
Not to be visually confused with strapon_over_chastity, which is when a character wears a dildo on top of their chastity device.