You can never go wrong with a classic Whoopee Cushion, especially if it is KING-SIZED! This whopper of a whoopee is 12" in diameter for your large-bottomed mark. "It's a real monstrosity of sloppy sounds and laughs. When the victim sits, walks, pushes, or falls onto the surface they'll release a bellows of bootie toots!"
One thing the designers of the large cushion didn't realize... most of your large-bottomed people already toot plenty on their own. And the more I think about it... plenty of people with small buns do plenty of booty boops, but what the heck, let's add to the fun!