I should have been more specific - I'm quite sure if you put a bare compact frame in a test jig of some sort it will be stiffer than a traditional frame for all the reasons mentioned. Obviously shorter tubes are stiffer than longer tubes. But since people don't ride bare frames, I assumed you'd need to include all the other parts that make up a bicycle

Let me be even a little more specific - I realize campact frames don't exist in a vacuum so as compact frame design was becoming popular other changes like shaped tubing, oversized tubing, new frame materials, oversized bottom brackets, and even things like outboard bottom bracket bearings also came along and all of these things improve stiffness. They are not, however, in any way tied to compact frames. Traditional horizontal top tube frames can be and have been built with all of those other features.