1) It should not be "bb". The tag should be "big_breasts", with the shorthand "bb" as an alias, like for all other tags.
2) "big_breasts" is a poor idea for a tag, anyway. Currently it ranges from "normal" big breasts to horribly deformed stuff like post #163076 or post #164060. Where is the use in that?
Let's try dividing the user base in three groups:
a) People who only like small or tiny breasts. To them, even a C cup is too large, so they end up tagging all largish breasts with "bb" (presumably so that they can search for "-bb"). But these people are already well served by the "flat_chest" tag (and "small_breasts", etc.), so there really is no need for them to tag the complementary set. Therefore, and considering that their perception is quite different from the average person's, I think they should refrain from tagging things with "big_breasts".
b) People who like breasts. It is normal for human males to be attracted to breasts (they are a sign of fertility, etc. - it doesn't matter why men like breasts, let's just accept that they usually do). These people will appreciate a search for "breasts" when they want to look at a nice pair. Small, medium, large, it's all good; in fact, people who like breasts usually display a preference for big ones. However, there is such as thing as "too much of a good thing": the average person doesn't want to look at deformed bodies and bloated balloons of fat. Therefore, it would be useful to have a tag for "grotesquely large udders" so they can be excluded from a search.
c) Finally, there are people who have a "huge breast" fetish and get off on stuff like post #163728. These people would be well-served by the same "grotesquely large udders" tag as before - they'd search for it instead of excluding it, but the set would be the same.
Now, we need to define how large is too large. Some people might say "larger than a D cup", or "unrealistically large"; while this might seem reasonable at first, we need to remember that this is a collection of anime-style drawings, not photographs. The normal proportions of an anime figure are quite different from those of a flesh-and-bones woman: longer legs, larger head, larger eyes, smaller mouth, thinner waist, etc. Making a direct comparison between anime breasts and real breasts is impossible: you get widely different sizes depending on which measure you take as the base (head size? waist size?).
For example, the wiki article on "bb" references post #154842. But in the anime style, those correspond to "normal large" breasts, not "huge" ones. It doesn't make sense to lump them in the same category as post #165533.
Therefore, I propose these changes:
1) No more "bb" tag.
2) A new "huge_breasts" (or "bloated_breasts"? You can suggest other names) tag for really oversize breasts, using a criterion based on normal anime proportions (*not* real life proportions).
Updated by lichifruit