I can kind of see the justification for distinguishing hyper_foo and absurd_hyper_foo (eg. proportionally building-sized, city-sized, or planet-sized boobs/balls/butt/etc), but wouldn't want to see it go beyond that.

The threshold I would pick would probably be 'body sized' -- ie. if you removed the specified body part, you would be reducing the volume of space occupied by the character's body by >= ~50%, that would be the lower bound of 'absurd'

Of course we would also have to expect that whatever distinction we refine for breasts will be at some point applied to other hyper body parts. Suppose we say absurd_hyper_* is a thing, then that would equate to at least 4 additional tags - absurd_hyper_{breasts,butt,balls,penis}