If you have followed the previous recipe, you will notice that when you use Play In Editor, the button that loads is unusually small.
The reason for this is UI Scaling, a system that allows you to scale the user interface based on the screen size. User interface elements are represented in terms of pixels, usually in absolute terms (the button should be 10 pixels tall).
The problem with this is that if you use a higher-resolution panel, 10 pixels might be much smaller, because each pixel is smaller in size.