iMac - Mycloud 4TB shows 90% full but this can't be correct

iMAc, running High Sierra 10.13.6

mycloud shows 90% full, but I have deleted hundreds of GB of old data and the % full does not change.

How to find and get rid of junk taking up space on the drive?

mycloud connected through ethernet to the router (Apple Time Capsule, internet feed from main Verizon router upstairs via ethernet)