To reproduce:
1) Create a new project
2) Add a Sphere to the scene (and make it bigger to see the issue better, for example, 5/5/5 in Scale)
3) Switch to WebGL
4) Go to Player Settings
5) Set resolution to 300x300
6) Build and run it
7) Open dev console in the browser
8) Run the following block in it"

document.querySelector('canvas').width = 300;
document.querySelector('canvas').height = 300;
document.querySelector('canvas').style.width = '900px';
document.querySelector('canvas').style.height = '300px';

Expected: The sphere gets stretched (or squished if you set style.width less than 300)
Actual: The sphere stays the same

Reproduced in 2017.4.0f1, 2019.2.2f1, 2020.1.0a1