In case you decrease the size of your window vertically the whole scene scales down. But if you decrease the size horizontally, there is no scaling and the scene just gets cropped.
I spent many hours creating this playground to showcase this: Babylon.js Playground.
On horizontal window resize the mesh always stays completely visible due to rescaling. On horizontal scaling the mesh does not scale.
My feature request would be to have it scaling in both directions.
I see the same behaviour as previously on that playground.
I managed to do it for my specific usecase adjusting the viewport in a way that it always fits a specific mesh either horizontally or vertically into the viewport.