QLayout::SetMinimumSize doesn't work

I have a MainWindow with minimum size 300px.
This Window contains a layout with 3 buttons, each minimum size 200px.

if I set this layout to setSizeConstraint( QLayout::SetMinimumSize ) I would expect that the window can not get smaller than 600px. But it can.
This should be the behaviour of SetDefaultConstraint as described in the help.

See attached .ui file