App icons are really the domain of the application developers themselves; Google simply provides guidelines. Unfortunately, that leads to icons with different sizes and an inconsistent look.
So Google is taking it upon itself to enforce the guidelines by normalizing icon sizes as shown by the above example image from Ars Technica.
I suppose Google could have taken a different approach and tried to enforce the proper icon sizes through app reviews but with more than million titles in the Google Play Store, that's a chore at this point.
Nexus phones use Google's standard app launcher and will automatically get updated with the changes. But you don't need a Nexus phone to use it.
By installing the Google search app (if you don't have it already) and the latest version of the Google Now Launcher, you'll get both the auto-rotation and icon normalization features. Note that you'll need version 22.214.171.124 of the Google search app; it's not yet appearing for me so it may take some time for Google to roll it out.