It's said in narration in the manga, and was most likely said in the anime as well.
"The original Majin Boo hadn't a chance at winning... After all, when they were split in two, the evil side received the majority of their power..."
I believe people made a fuss back when VIZ translated this volume, because they simplified the quote. It's wasn't clear that the evil Boo took away any of the fat Boo's power in their translation, I think. Can anyone post the VIZ scan or write out their "interpretation" of the line?