This is a problem related to the WebDAV client implementation on Windows and nothing we can directly influence. This is just the way, Microsoft wants it to be:
“Research on WebDAV protocol RFCs and verification from Microsoft’s product team confirmed WebDAV protocol doesn’t support querying server’s disk capacity” - Microsoft
Microsofts “product teams” seems to be very bad at research, as apparently they haven’t heard of RFC 4331 (which Cryptomator has implemented) and thus Windows just shows the capacity of the system drive.
The good news is: The shown capacity doesn’t affect how much files you can actually put onto the drive, as the capacity doesn’t change. So even if you have just 10 GiB left on your C: drive, you can still encrypt 100 GiB, if your vault is located on e.g. the D: drive, which has 100 GiB left.