Today I noticed that many VMs do get a lot more RAM than they actually
use. While using only about 200-300MB small vms like -net and -firewall
get gigabytes of memory and this seem to be the case even if memory is
running out (sum of all VMs approaches physical RAM size). Also dom0 is
using only about 700MB but gets 4GB.

1) does memory balancing take back memory from a VM at all?
apparently, as much as there is enough, each appVM gets MaxMem-size
(kernel param, usually 4G). When memory is gettng tight the qmemmman
manager does "balooning" whatever that is exactly.
This behaviour might be linked to errors (e.g. my qubes install does not
support 5.x xen kernels: crashes can be caused by "memory stress" and
even if not, they always finish by loads of qmemman log entries, before
deep freeze (not even a kernel panic, just sudden death)

2) how does it happen that VMS get assigned this ridiculously larger
amount of memory compare to their usage?
by design, as explained.

3) is there something that can be done besides manually setting limits
for all VMs?
Good question.

I current think about limiting all small VMs to 256MB and dom0 to 2 GB
of RAM (by GRUB parameter) lacking any idea for a better approch.

Tell us if that works! My qubes has no grub. But you can set kernel
params in /boot/efi/EFI/qubes/xen.cfg

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