Hi Michael,
Thks for this clarification, I had not realized the lack of supported compress programs on Windows. Although I did successfully install bzip2 on mine this morning, I'll acknowledge a typical install would miss it.
Pscp does not support multi-hops, so it's a longer process for the user who needs the file to be copied as root. Worth it for a 100 MB file, but not so much for a few MBs. I have no idea how much compression improves the process, but if it was implemented in Tramp, it's probably because it's worth it?