When Porsche was originally designing the Boxster and the 996, the design teams worked in parallel, using the same front ends. This saved a lot of expense, and makes the parts supply easier to maintain. From the firewall forward a Boxster and Carrera 996 aren't that much different. The doors are the same too, I think. In the back end, it gets completely different - Boxster with a mid-engine and 996 with engine behind the wheels. The carrera's larger engine, more torque, wider track, and rear location need bigger wheels/tires to keep everything planted and under control.
Hope this helps - I was a couple of cars behind a Carerra cab last weekend in our club drive - LOTS of rubber back there!
