We have Hyper-V running on Windows Server 2016. We created new VM with virtual disk size of 10GB and boot it with corresponding Linux ISO.
The Debian installer and the old Ubuntu Server installer, report the total disk space as 10.7 GB. The same happens when we use VM generation 1 and 2. Tested with Debian 9.6.0-9.9.0 installer and Ubuntu 18.04.2 LTS alternative (old) installer.
The new Ubuntu Server 18.04.2 LTS Installer (Subiquity) reports the correct size of 10GB.