Originally posted by Randman
Basically just because of more weight at the bottom of the car. This is why you dont run lead weights on the top of the shock towers 
I think you need to consider the rest of the car parts
trust me I know a bit about engineering
The extra thickness of the chassis will add more mass but it will all be above the top surface of the original chassis, so the CG of the chassis alone is higher for the thicker version, then you have the rest of the bits that sit on the chassis and they will also be sitting higher, only a fraction of a millimeter though, so they all raise the CG too.
Think of a 5cm cube and a shape thats 5cm wide in both directions but 6cm tall, which has the higher CG if they are made of the same material?