Hello hayoh,
It sounds as though you're seeing your storage usage increase while not downloading content on your Mac. Apple Support is a great place for help! To clarify, are you noticing the storage increase when creating documents such as a Pages document, PDF, numbers, or other information such as that? This can also include using PhotoBooth. performing tasks in apps, or updating documents can cause the storage usage to increase.
If you'd like to manage the storage on your Mac you can find some great information in the following article.
How to free up storage space on your Mac
If you continue to have issues, or notice the storage increasing without use you may want to reach out to Apple for the best support. You can find their contact details via the Get Support website.
Take care!