Ok, I "solved" it. But it should not act this way...
I had added another variant (1300x700) and left the original (600x600). As it seems I then accidently switched to the standard one (600x600) and this caused the "problem". Switching back again to 1300x700 did it (I have deleted the unused now).
I have mentioned another issue:
Opening the project (B4J is started fresh), starting the designer and load the layout will show this. Variant is 1300x700. Looks like it starts with 600x600 anyway...
Switching to "match device" and back to "match chosen variant" shows the correct layout:
I had expected it from the start. Bug or is it me?