There was a guy who spent a few years correcting the size tags on images, and would always post the following as an explanation below any picture he corrected:
big breasts < huge breasts (when each individual boob is about as big as the character’s head) < impossibly large breasts (when each individual boob is about as big as the character’s ribcage, or bigger).
I believe “hyper breasts” was created sometime after the other three, so its boundaries are the fuzziest, and in fact the tag descriptions are the opposite of the tag implications. The tag descriptions say that “hyper” (bigger than the character’s torso, basically the old definition of “impossibly large’) is supposed to come in between “huge” and “impossibly large” (which is now defined as big enough to prevent standing normally). But in the days before the “hyper” tag, the tag implications were set up so bigger sizes also implied all the smaller sizes. “Huge” implied “big”; “impossibly large” implied “huge” and “big”. And if you look at the tag implications now, “impossibly large” still only implies “huge” and “big”—while “hyper” implies “impossibly large”, “huge”, and “big”.
So the tag descriptions are saying the hierarchy is: big breasts < huge breasts < hyper breasts < impossibly large breasts.
But the tag implications are instead saying: big breasts < huge breasts < impossibly large breasts < hyper breasts.