Ran a quick check using the polyfill, firefox and the MR emulator - the resolution you start the canvas at is the resolution that will be used in the XR experience. This does make sense, as WebVR required changing the size of the canvas to the right resolution before entering VR. But as we are in XR, we don’t resize the canvas. As a temporary solution enlarge your scene to max before entering XR, it should give you proper resolution in the headset.
What you can also do is force the canvas to resize (the actual canvas resolution). The only way I am going to introduce this kind of step in the framework is if it is impossible to achieve that on the dev level (which i doubt). Adding canvas resizing just for the WebXR polyfill will probably not integrate in the framework.