LiquidEarth stores downloaded Space data locally for two purposes: faster loading through the quick-load cache and access to Spaces that have explicitly been made available offline.
Quick-load caching occurs automatically when Space data is downloaded. Under Settings: Data, Maximum Space [GB] sets the storage limit for this cache, while Used Space shows its current size.
To retain a Space for use without a connection, explicitly set it to Available Offline. The Offline Projects > Used Space value shows how much storage this downloaded data uses. Spaces accessed in Offline Mode (Experimental) should be treated as read-only because offline changes do not synchronize with their online counterparts.

The Data settings show offline-project storage and quick-load cache usage.