Letting the Layout fill the whole area of the parent

[draGy]

Hi,
I am new to Qt and I am using the Qt Creator.
I was wonderin how I can set a layout into a Frame/Widget (e.g. Tab Widget) which automatically expands to the widgets size?
Like the Top Level Layouts do.

When I place a Layout on a Frame/Widget the layout always has a fixed size and if I resize the Widget/Frame the layout does not resize equally. It stays static.

Isnt there a way to tell the Layout that it should always take the same size as the parent Frame/Widget has?

Basically just want the layouts to behave like the top level layouts behave when they are on the mainwindow.