Hazarding a guess, the zip files are automatically created so if you are sending a bundle of 30 files it makes sense, but if you are sending only one (and it only saves 1% of the size) you have no human to intervene and skip that bit of the process. I've also had email systems that mangle files (even pdfs) unless they are zipped. So their thinking is: better safe than sorry, and by the time you've downloaded it it's no longer their problem!