Ok, this makes absolutely sense to me - I did not know, that this is the way the boot sequence is working.
So I tried once again and just cloned the whole disk - ignoring the different physical sizes -
It worked!
I expected to get some error saying, that the process cannot write any more blocks on the target disk, because that one is smaller than the source. - Excatly that happened, but did not have any consequences regarding the usability of the resulting copy.
Thank you all for your input to this general topic!!