I'm flagging this because I think it used to work properly however I could be 100% wrong. I have a 150GB sparse img file on my cache for my Windows VM. The Main tab of the UI is showing disk usage for the full size of the img file despite it's actual size.
du -skh vdisk1.img
92G vdisk1.img
ls -lh vdisk1.img
-rwxrwxrwx 1 root users 150G Jan 23 01:57 vdisk1.img*
Running a du against cache I only get 129GB used which is more accurate.
root@nicknas2:/mnt/user/domains/Windows 10# du -skh /mnt/cache/
129G /mnt/cache/
Again, I don't know if Unraid has always done the calculation this way but I don't recall my used size on cache being that high until recently with 6.10rc2.
Feel free to close if this is expected behavior but perhaps it's worth looking at for enhancement. Would be good to show actual size used.