size[policy] of nested QMainWindows

I've got a QMainWindow in a QTabWidget in a[n otherwise 开发者_Go百科empty] QVBoxLayout in a QWidget in a QTabWidget in a QMainWindow.

But the size of the internal QMainWindow is being reported as 640x480 (the size of the external QMainWindow) when the sizeHint is reported as 70x26 (smaller than the widgets inside the internal QMainWindow). It looks like 70x26 to me.

How can I adjust the size to stretch to fill the internal QTabWidget/QVBoxLayout/QWidget?

The internal QMainWindow has a QTextEdit for the central widget and a QComboBox in a QDockWidget on the bottom docking area. The external QTabWidget has other QWidgets which work fine.