I believe I've solved this problem by customizing the Root Node of my Scene.
The pre-requisites here are that your project settings are set to:
- Display/Window/Stretch/Mode: 2d
- Display/Window/Stretch/Aspect: Expand
Then you'll need to make the Root Node in your Scene a "Control" type node.
At the top of the 2D editor, with the Root Node selected, click Layout:
- First select "Full Rect"
- Open menu again and select "Center"
There are some other options in there for you to play with as well, to get the expansion to happen differently