The disk space that will be used depends on the assigned policies. With the Default Policy, objects will be mirrored, i.e. 10GB of data will use 20GB (10GB on each host). Other policies will use more, or less disk of the available space. Since vSAN doesn't know which policies will be used, it shows the complete available disk space.
I am using the vSAN default storage policy to handle 1 failure. It looks like it is just doubling everything. So it reports that my available storage is twice what it actually is because as I start adding in more space, ir copies the data to both nodes(mirror) and would fill when it gets to 50% of what it reports. So it reports the VMs are twice the size they actually are, etc. I guess I am just use to dealing with actual numbers and views based on what RAID policy is applied, etc. Thanks