It absolutely has to do with screen size.
From this page:
Control when to turn on mobile layout
The app supports two styles of layout based on the screen size. One is for desktop and one is for mobile devices. When either the height or width of a screen display is less than 600 pixels, the mobile layout applies automatically. However, this can result in unexpected behavior when the app is embedded in a website, for example, when the pop-up in the website is in the mobile layout style. To control the layout style, use mobileBreakPoint=<pixel number>. For example, you can remain in desktop style until the screen size is less than 300 pixels, as shown below:
http://<your portal url>/apps/webappviewer/index.html?mobileBreakPoint=300
Have a great day!
Johannes