Proxmox tries to allocate only as much as RAM as a VM currently needs through the help of the ballooning driver. While Linux has them by default, and Windows needs them to be installed through the Qemu Guest driver, I couldn't find much info whether there are any (native) ones for Mac OS.
Nonetheless, using PCI Passthrough means that the VM will automatically use 100% of the assigned RAM because the hardware (drivers) might randomly access parts of the RAM that would be potentially unmapped.
From what I understand, even with ballooning drivers, mapped memory would never be released and quickly fill up. E.g. when tested by the OS at startup.