There are two possibilities:
1. Some kind of spam, lots of times these say something like "you've used up free storage, click here for premium/to avoid your account being closed", and WHAMMO it's a virus/phishing/other bad thing.
2. Do you also have a lot of photographs stored somewhere -- is there a OneDrive folde? If it's legit, then I think the 7GB doesn't count only hotmail/outlook storage, but also stuff in a (free) OneDrive account. Lots of photos or videos would go towards this. Go to File Explorer, and check the bar on the left to see if there's a OneDrive, and if so what's in it.