Well actually, it pretty much is. We did this a few months ago and I did a load of insurance quotes demonstrating the myth that its not about the size of the car and the engine that you can insure a Mondeo for the price of a Fiesta is just that - a myth. Sure I take the S2000 example but its an obvious outlyer - its power output is considerably larger than most 2 litre engines. Generally, in the cheap car market, power output and engine size are roughly proportional. This will change one day when the current generation of cars depreciate but at the moment it isnt really the case.
Generally speaking the smaller the car and engine, the lower the insurance bill.