Hi Luki,
By default, Hyper-V sets each VM’s maximum RAM to 64GB. You might want to set the maximum RAM to a lower value if you don’t require that much physical memory for some of your VMs.
Please read through article below which should provide more insight into the same
Virtualization: Optimizing Hyper-V Memory Usage | TechNet Magazine
Hope this helps.
Regards,
Deepak Kori