Friend this gives to the fact of the type of disk they chose when creating the server.
See, the Hyper-V te 3 options fixed, dynamic and differential.
Fixed
In this format it will already allocate all informed space from Host HD.
Dynamic
Here it will allot space as your VM grows.
Differential
In this it is necessary to already have a disk file that we call father, the father will not have modifications, everything you do again will be created in a new file. (Used mostly for labs, for example, you want to set up a lab with 3 windows server, to occupy less space creates a file only with the S.O. installed and it will be your parent disk, so create 3 VM with differential disk type and point to the parent, so you will only have 1 OS consumption in place of 3.
In your case this Pfsense was mounted using the fixed, which is normal, since the fixed offers more performance than the others.
To learn more about the subject take a look at this link.
There are ways to decrease (Shrink) the size of the disk, but you would have to first enter the VM, decrease the size of the disk inside (in Windows done by "Disk Manager" and in Linux could be done by Gparted") and then Edit the disk by Hyper-V using Shrink. This link shows with images how it can be done.
But be very careful, because by decreasing the size of the disk you may end up losing some information and corrupting the same
thank you very much for the reply. helped a lot
– cpdtexto
@I’m glad you could help. If the answer was useful to you, would you kindly consider branding it as correct to help others with this same question?
– Murilo Portugal