My guess is that it could have been one of two things:
The installer files for the new version. Uncompressed, they're probably about that big. The files would have downloaded and uncompressed automatically in the background before you actually started the upgrade, so that's why you might've seen the extra 7.3GB.
Or, it could be the actual OS files.
When you do the feature upgrades, Windows actually downloads the whole OS and installs both versions at the same time. When you restart, it makes the new version the default, but the old one is still installed (just disabled).
The old one is kept for 10 days, so you can roll back easily. Since the old version is still technically installed, it's pretty quick to switch back to it. So if it's been more than 10 days, it might have been the old version was removed automatically.
09-17-2017 02:51 PM