I've noticed that huge_breasts is getting tagged on a lot of images that are clearly not "a single breast is larger than the character's head". (Examples: post #2138965, post #2138760, post #2138271, post #2137940, and that's just the first page of results). large_breasts seems to be being used for things smaller than the wiki says, too. Is this just a common tagging mistake, or should the definitions on the wiki be revised to match actual practice?

It's just inconsistency. In my opinion, a character like Seaport Hime has huge breasts but she's mostly tagged with large breasts.

Very few characters canonically meet the wiki definition of huge_breasts. My back-of-the-envelope calculation is that the minimum size for a single breast to be larger than the owner's head is a (U.S.) G cup.

I generally use huge breasts as being head-sized or larger, since anime heads are already larger than a typical head to begin with.

Maybe change the definition to "larger than face" since a character's hair/hairline makes it hard to distinguish the head size?