what I wrote above:
What probably happened (it happened to me several times) is that the update process didn‘t find the location of your official folder.
I don’t know why this happens, that question would need to go to the developers. But it does happen. Then the default installation path is suggested. If you told MSFS at any point in the past to install to any other location than the default one I bet that you’ll find your original installation where you placed it and the new one in appdata. If you have never told it to use another path the whole thing becomes really mysterious and I can’t tell you what happened.
Everytime you start the sim it checks if its files are complete and in line with what the online service asks for. If not it will tell you that you have to update, regardless of your current version. It will want to update your installation with the required files. If there are none at all in the location where it thinks the files should be it will obviously want to install the whole sim, 122GB in this case.