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 have found another topic on this: Scale on resize - Questions & Answers - HTML5 Game Devs Forum. But it looks like there was no final solution?