It is important note that geometry() will return a proper position and size only after the widget is displayed (i.e. show() is called and painted)

I have made the widget as big as the window will go and it remains proportional to the size of the window when I resize the window
Does that mean you have one widget on a top level widget?
When say resize and propostional, it kind of implies that you are using some layout manager. As I said earlier the layout manager will resize (change the widget size) to fit it into the parent window/widget, and will also add the margins. These margins will cause the widget to shrink.

If you are using layouts then use setMargins(0);