I have a twiggy and a box. They are both roomy and gorgeous bags. I'd compare the box to a toaster and the twiggy to a wide loaf of bread. Those might not be good comparisons! *L* Basically, the box is short and fat and the twiggy is long and fat.
There is a pic of my box and twiggy side by side in the "documenting balenciaga colors" thread. I love them both, but the box size is better for the amount of things I carry.