The MobyLinux Hyper-V machine is provisioned on first start of docker on your computer.
There is no way to change the original 60GB size by configuration, how is stated inside the C:\Program Files\Docker\Docker\resources\MobyLinux.ps1 (powershell script):
# Hard coded for now
$global:VhdSize = 60*1024*1024*1024 # 60GB
So, the solution was to change this line to
$global:VhdSize = 120*1024*1024*1024 # 120GB
This worked for me. You need however to reset your docker, causing it to provision again the hyper-v machine.