I think fat became sexy as part of the hip-hop cultural influence, along with glorifying crime, anti-police sentiment, "fashion" and of course, the dumbing down of the English language where the sub-literate introduce the new slang and all the dumb kids adopt it. Remember "phat" being a good thing?

Now, idiots use "thot" which is an acronym for "that hoe over there" which makes no sense at all in common usage. (One shouldn't say "that thot" because it's redundant, for one minor example.)