I have a WebGL game created by Unity. It is working fine when using a desktop browser but the width and height of available window space is not filled when hosted on Facebook Instant Game. In fact, the whole game is scaled proportionally to 16:9 ratio and put in center in the window. The included picture shows the current behaviour. When running outside Facebook in a desktop browser all window estate is used.
In this picture, you can see that the game doesn't use all the window spaces. The same game running on a desktop browser is filling all the available space in the browser window.