[FenwickHockey65]

This is hard as hell to explain without an engineering background. Number 3 can do a much better job, but I'll try.

Basically, a platform is the basic underlying architecture of the car. GM has a bunch of them, from FWD compact platforms to FWD/AWD midsize platforms to RWD truck/SUV platforms, etc. Platforms are designed specifically for one drivetrain or sometimes to incorporate AWD as well. The car is basically built up from the platform.

As such, the platform limits what you can and can't do with the design/engineering of the car. All of these complexities are things that I have very little knowledge about, so I won't even attempt to explain them.

However, there can be variants of a platform. For example, the new Epsilon-II FWD midsize sedan platform has spawned at least two, maybe three different variants, a SWB platform which underpins the Opel/Vauxhall Insignia and 2011 Buick Regal, a LWB platform which underpins the Buick LaCrosse/next-gen Chevrolet Malibu, and supposedly a premium variant that the XTS Concept is built on. This holds true for the Alpha platform, as the 3rd generation CTS will supposedly ride on an extended version of the ATS's Alpha platform.

When we say the Alpha platform will be smaller, we're comparing it to the current RWD Zeta architecture that underpins the current Camaro. Cabin space is something that we know nothing about for now since there aren't any existing cars currently on the Alpha platform. However, there are ways of engineering the car so that cabin space is maximized even on a small platform. Take a look at the 2011 Chevrolet Cruze for example. It's built on the FWD compact Delta-II platform but has interior space rivaling some midsize sedans (which will be a major selling point for it).