Honestly, my suspicions of this is that as they grow outwards they lose the attention that they used to get when thin. Losing this attention then pushes them to try and fill in some "diversity" points while filling out sideways. This often stems into the LGB community, usually with some made-up sexual attraction or with the T add-on. Notice how wide the acronym has gotten as gays/lesbians/bisexuals have become more accepted in society. These beasts need to stand out from the crowd. So they keep adding new things onto the label (most of which make zero sense), but have the validity/societal acceptance that LGB movement has achieved.