Still, it is a fact of life. All things being equal, your statement is certainly true. The problem is that there are a lot of variables - compression ratio, gearing, throttle losses, etc. If every chassis were tried with every available engine the most efficient would probably be one of the smallest, but we would find a lot of bumps in the graph of economy as a function of power or displacement. For example, the EPA MPG ratings
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for the BMW Z4 roadster 6 speed 3.0L is better than for the 5 speed 2.5L. Probably the improved gearing doing that, but there we are. In theory, gasoline engines (otto cycle) have a thermodynamic limit of efficiency around 65% while diesel engines (diesel cycle) have a limit of efficiency around 50%. But since those limits are approached as the compression ratio approaches infinity, the diesel wins almost every time.
Mike