Potentially... first, are you using the ECM backup task, or using SQL backup alone? (we use sql backup alone, fyi) If you are using the ECM backup task; that's ok; but you might be encountering vssadmin allocation issues.
If you haven't ever looked at / managed your volume shadow service allocations (vssadmin). https://www.anoopcnair.com/configuration-manager-sccm-backup-uses-the-volume-shadow-copy-service-vss/
That bit me once about 12 years ago; had to really examine vssadmin list shadowsstorage, and do some adjustments.
That might be what is "taking up space"; when you can't easily "see what is taking up space" when looking just at normal / visible files on that drive. the VSS might be legitimately taking up space; you may want to check if that is the space hog. if it is, it's a relatively easy fix; you just have to plan out where you want your shadowstorage to be (instead of the drive with your database files on it).