It’s linked to the way how the game draws two separate pictures that are shown to each eye.
You have two kinds of IPD (inter pupillary distance) working here: the real life distance between your two eyes and the distance between two images (camera IPD) that are drawn in the game. If these two distances differ from each other your brains will see a “world scaling error”. Ie. If the game draws the 2 camera positions too far from each other compared to your own real life eyes you will perceive the world as being too small. If the game draws them too close you will perceive the world too large.
Currently MSFS has a fixed game camera distance that makes people perceive the world too large or too small if their real life eyes wont match the fixed game inter pupillary distance.
Here is a thread about the issue:
I have no inner knowledge about the workings of OpenXR but I bet there’s a way to move the software camera distance through OpenXR giving us the ability to match the game camera distance to to our real life inter pupillary distance to match the world scale we’re used to. A small offset variable to fix the problem.