You are right that the screen just becomes wider, but the monitor distance works with percentages of monitor size. So 100% for 4:3 is only 75% for 16:9 etc.
What you are asking for is a ratio conversion, which I've been back and forth on including or not. To but it simple, I don't want to make this whole thing too advanced, there's enough options already But quite a few people have been asking for it, so I'll go back to the drawing board!