Yes because you are previewing html on your desktop and mobiles are quite different in almost every aspect.
Layout size - Size of let's called this "area" where your game happens - think of it as a level size
Windows size - Size of your game window. How big, in pixels your game will be on preview and after export. For example 1280x720 pixels means that your game window in browser,mobile or desktop will have 1280 pixels wide and 720 pixels tall.
I'm always testing it on my S3 with Window Size 1280x720 (layout size does not matter) and I'm using "Unbounded scrolling" with Scale outer.
But you need to do some manual sizing and positioning for your objects to work correct.