I just did an auto-update from 0.15.37 to 0.16.6 on Windows, and the total download bandwidth used was over 1.4gb (it only showed one decimal place). I used NetBalancer to monitor usage of factorio.exe.
The 0.16.6 Windows zip is ~825megs (quick start/stop download, didn't check the actual size), so either the update packages are including redundant files, or some form of a "smart downloader" would be appropriate to calculate update packages' total size versus the full zip, and have the auto-updater take the route of less bandwidth, for benefit of all.
This was something I wanted to bring up since I know many countries' ISPs have some form of bandwidth cap. I'm not complaining in the slightest--I'm speaking out for those who DO have slow ISPs and/or bandwidth caps.