How much RAM can be reservated for a single VM

Hi,

I have a 6.5u1 standalone host with 10GB of RAM, and one VM on it. As there is a passthrough device assaigned I would like to reservate as much as possible RAM to this VM, but as soon I increase the reservation above 4048MB  following error appeares when trying to start the VM:

The available memory resources in the parent resource poolare insufficient for the operation

I tested this also with 8GB RAM, there I was only able to configure 3GB reservation.

So why is it not possible to assign more reservated ram to a VM?

Thanks,

Crispyduck