the loading is good but sadly in mobile is not show full screen on portrait mode, you can play game here to see how it show in your mobile
Viewing post in (Unity) Better Minimal WebGL Template comments
The template is scaling the game as expected: It's scaling it down such that the whole canvas is visible while maintaining aspect ratio.
If what you're expecting is for it to automatically fullscreen or lock into landscape mode to make better use of the device's screen real estate, that's something which should be handled by code specific to your game or by the site embedding it (e.g. itch has orientation and fullscreen settings on the project page).