When you say "breasts should be X times larger than the character's head to qualify" (where X is an integer equal to or greater than 1), do you mean BOTH breasts' size combined, or each breast on its own? Because throughout my time surfing this site, I've seen pretty much each of the tags in the topic title being applied BOTH ways.
E.g.: some pics (like post #1193665, post #1121344, and post #205324) with the "huge_breasts" tag may have the character's breasts COMBINED be "larger than but no more than than twice her head"; others with the same tag (post #364019 and post #930300), however, would instead have a character whose breasts are EACH "larger than but no more than than twice her head", and COMBINED would be easily (and sometimes WELL BEYOND) "twice as large as her head". Note that if the first batch of pics were to follow the "each breast alone" interpretation, they would have to be retagged as large_breasts instead, since each breasts is "roughly similar in size" to the character's head; conversely, if the second batch had followed the "both breasts together" interpretation, they would have to be retagged as gigantic_breasts instead.
And this even extends to the "gigantic breasts" tag: pics like post #1175154, which despite featuring only ONE girl, has both the "gigantic breasts" and "huge breasts" tags. The former tag was most probably added by someone operating under the "both breasts" interpretation, while the latter one may have been added by someone taking the "each breast" interpretation instead.
Updated by Serlo