I think comparing container images would be a good start. Use the most minimal base image you can get away with, or even "FROM scratch" if possible, but compare only images with the same architecture. I'd prefer 32-bit, to take things like running 32-bit on 64-bit or 64-bit with pointer compression out of the picture.
Then compare the size of the uncompressed exported tar. Probably also not completely fair, depending on what question you want to answer but it takes the obvious variances out of the equation.
EDIT: Thinking about it some more, it might even be more fair to compare maximally compressed image size to account for compression within the container. Of course you'd have to compress with the same algorithm and parameters or just add the decompressors size to the final result like they do in compression benchmarks.