How does the screen resolution and screen sizes work in g develop?

I’m so confused about this because I have it resized to 1920 by 1080 and the width of the screen is set to fit the screen. When I go to preview the game it’s just fine and everything looks the way I want it to, but if I set it to full screen all of the sudden the width of the screen is too big meaning I can’t see the very right side of the screen and vise versa . But if I export it as an index.html file and do a live preview, I can then see that the width of the screen is too small meaning I can see the outsides of the screen which I don’t want. How does this work I really don’t understand?

This is what I usually do to solve it.

Tho I’d like to see how the resolution of your game supposed to look like and when it doesn’t appeared as you wish.