But we don’t usually count the decompression program size when evaluating compression ratio. Eg 7-Zip is about 1 MB, but you don’t think about that when evaluating particular 7z files.
But sure, it's a constant factor, so if you compress enough data you can always ignore it.
But I do want to point out that almost everyone installs at least one multigigabyte file to decompress other files, and that is the OS.