Nitpick: Doing it exactly like this is flawed because you let the compressibility of your references taint the result; what you would prefer is the compressed size of testfile given sports.txt/... as a dictionary without accounting for the compressed size of that, no?
You're right, you should subtract off the compressed sizes of the respective reference files before comparing. (This suffices if we assume that later input data does not influence the compression of earlier input data, which is true except for certain unusual conditions like a repeated substring at the end of the reference data that also appears at the beginning of the test data.)
Really interesting approach though.