To me there's a definite difference in between breasts that are within the spectra of what is considered normal up to uncommonly big, and then the range from sizes that do exist on the very end of the bell curve, but only on the very end, up through medical oddities and beyond into pure fantasy.
Of course it's going to be a grey area as mentioned, but at the same time head size is indeed a quite good border line for me. That's as large or even larger than those last few percent in the real world that do statistically exist.
While I know it is quite short, the original purpose of this suggestion is for the people who is instantly turned off by them (such as myself) and from what I've seen (that passed through my mental filtering/merely curious) is that they aren't really used in sexual stuff, like inflation or just comically big without any focus (quite rare tho).
So anyways, my point is, it's less of a description and more of a criteria/classification/consensus for determining which goes where (as in the big breast tag, it said exaggerated size but didn't tell by how much, leaving room for subjectivity, where when I think about it again, it's alright ig)