Two App Screens

To the best of my knowledge, you can't. That, or I'm drawing a blank. You can jump from the app you're using to the Watch Face by double clicking the Crown, assuming the Watch Face was your "last used app" but a single click when in an app will bring you to the app screen. Not sure what exactly you're seeing with this so-called second app screen, to be honest.

Edit: Ahh, I technically found a way that you could single click out of an app and go right to the Watch Face, though it is not really applicable to what you're asking- in that it is a very rare case scenario and would not work or be used. The scenario is: you start on your Watch Face -> you double click and are taken to your last used app. -> you single click out of that app and are taken to the Watch Face. Again, just pointing out that there is a very rare scenario where a single click would take you there. This has little to no use though for the true topic, as far as I'm concerned here.

 

Did you turn on accessiblity or something?

 

This is the Zoom mode under the accessibility options.
https://support.apple.com/en-us/HT204576

 

It's the Reduce Motion setting in Accessibility that you need to turn on to have all app icons appear the same size. When it's turned off, the icons resize as you scroll.