This may relate to records that have been deleted. Knack keeps unused assets in case backups are ever needed and you request a restore via support. I just ran the delete process on my sandbox app and it dropped from 99.6MB to 10.4MB because of items that have been deleted from the builder, but are still stored server side for restoring purposes.
If you go to the apps section and click on the storage value you will get the popup dialgue to allow you to delete unused assets. This can take some time depending on volume. If the items have been deleted from the builder its highly likely you don’t want to restore them in the future. Once you run this process those deleted items will no longer be restorable.
