Question: how does majin buu transform?
Answer: When Buu expels the evil inside him, it takes on a taller and much skinnier, frail form referred to as "Pure Evil Majin Buu" (魔人ブウ 純粋悪, Majin Bū Junsui Aku). When this Buu absorbs the fat one, he transforms into a muscular version of himself, referred to as "Evil Majin Buu" (魔人ブウ 悪, Majin Bū Aku).