The first time I ever heard about "Baby Got Back" by Sir Mix-a-Lot, I was about 13, and I woke up to a Cleveland radio DJ talking about it, approximately 6 months prior to the actual release of the single. He explained that Sir-Mix-a-Lot was planning to tour the country with a giant butt-balloon in support of his new single, which I heard him say as "Baby, I Got Back (I Like Big Butts)."
I pictured "Baby, I Got Back" as a concept of someone returning home, for example,a husband returning to his wife after a long trip. So it followed that him declaring upon arrival, "I Like Big Butts," was indicative of his realization that while he was away he underwent some type of metamorphosis, and now that he's back, he's essentially saying "Honey, I hate to break the news to you, but something happened while I was away, and now, I like big butts."