Why do Gzipped source size is better? I mean, if with the same number of LOC, Gzipped size is different, that means one program is more repetitive than the other, right? I suppose experiments were done to show that Gzipped size was "better" in some sense?
I don't think you can objectively measure readability. It is tightly coupled with concrete syntax, which is highly subjective.
Because gzip (or any other text compression protocol) tends to reduce the size of the file without reducing the information content. So for instance, if you used longer variable names but the same overall structure or if you split single lines up in to multiple lines (or vv) the gzipped bytecounts would be relatively close in spite of huge differences in source size.
I don't think you can objectively measure readability. It is tightly coupled with concrete syntax, which is highly subjective.