If I’m not mistaken, space is allocated on a per-service basis, not per-domain. Space usage is displayed on a per-service and per-user basis. When you create a domain, it’s created under a certain user, and that user is created under a certain service.
Under the Billing: Services tab in the Control Panel, you’ll see something like this under the details for any particular service, like Code Warrior: “MB of Storage Space - 43.852 out of 150.”
To see which user is using what proportion of space, look at Users: Users: and you’ll see something like: “username - shell - machinename - /home/username - /bin/bash - 0.617/150.000 MB”
In other words, storage is spread out over the entire service, and how you allocate space to your users and their domains is up to you.