It looks to me that you have the sash width set to 50% which is what the plan preview is showing.
You should find the elevation and 3D previews will be the same.
The diagram in the dialogue is just an example and not a true to size image.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11