Thanks for the explanation.
I see the problem with a single vmdk file now.
I did not understand the reasoning for making it the default storage before, thought that it had to do with storing the files on FAT32 formatted disks or similar.....
What I did in the end was that I copied the whole virtual machine folder (after shutting down the guest) to a USB3 disk with 400 GB free space.
Then I connected it to WorkStation7 and used the expand function as you suggested.
The virtual machine copy took 12 minutes or so, not bad at all.
But the expansion function in WS took several hours. After this I could easily expand the disk inside Win7 to use the full drive space and I now have a 250 GB virtual disk.
But in the end the vmdk file turned out to be 105 GB and after I installed RAD Studio XE7 it is now 121 GB on the USB drive, so the virtual machine fits on my laptop drive with about 30GB to spare, provided I delete the original vm.
I have to get rid of the vm's I use less often to free up some space there since 30GB is only 3% of the total drive space. I would like to keep some headroom.
So for the time being I will continue to use the external USB disk when I work on the new development IDE.
But it does work. 
Thanks again!