Got a big butt because it's got a big engine. A V16 with multi valve (in other words large) heads. So the engine is loooooong. It's also installed sideways, making the car wiiiiiiiide. It was initially partially financed by Georgio Moroder, who wrote lots of theme tunes for Hollywood movies. Wasn't the styling by Gandini, the man responsible for the Lambo Miura and Countach? I believe he was also styled the Diablo, but his design was rejected for being to similar to...... the Cizeta!
I seem to recall that in the early '90s the Testarossa - like horizontal slats on the side ducts were replaced by one vertical slat. Finally, I think the car can still be bought, brand new? Isn't it being made to order in California?