You are only considering the efficiency of scale and not taking into account new technology. For example, a 1965 VW bug had a 1600cc engine that put out about 40 hp and got about 30-35 mpg. Yet today it is routine to get 130 hp and 35-40 mpg out of an engine of the same size.
There's certainly something to be said for computerized engine management. There are, unforutnatley, thermodynamic limits - probably the most salient being that a Carnot cycle (which cannot be built) is the upper limit on engine efficiency.
I've always thought that more effort should be put into designing a good stratified charge system. It would allow higher compression ratios by increasing the "apparent" octane rating of this miserable quality gasoline we have to use these days.