Widget does not resize correctly

[Demandred]

Hello everybody.

I have the following layout in my Widgets:

Widget 1:
A QWidget (named extWidget) and a Groupbox containing pushbuttons in a vertical layout. Groupbox has max height. QWidget fills the rest of the screen: sizePolicy(expanding,expanding).

Widget 2:
9 pushbuttons in a GridLayout.

Now when the first Widget is created. I create the 2nd Widget with the extWidget as parent.

Now when I run my code the 2nd widget gets displayed in the extWidget area of the 1st. So far so good.
But the 2nd widget is displayed with its size as set at design time and when I resize the 1st widget, the 2nd does not change.

How can I get my 2nd widget to alter its size to the available space of the extWidget area of the 1st?

// Code from the constructor of the 1st widget
    m_ui->setupUi(this);
    // Create the 2nd Widget
    StdConfig = new unitconfigStd(m_ui->extWidget);
    StdConfig.show();

// Code from the constructor of the 1st widget
    m_ui->setupUi(this);
    // Create the 2nd Widget
    StdConfig = new unitconfigStd(m_ui->extWidget);
    StdConfig.show();

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Thank you for your time,
D