Here are some comparison pictures, to see how small the BubbleBum really is.
Here is the BubbleBum sitting on a RadianXT. The Radian is 17 inches at it's widest point. The BubbleBum is about 12 inches wide.
Here is the BubbleBum sitting in a Graco TurboBooster. The backs of the 2 boosters are even, so you can see the difference in leg support.