You may need to write different code for different CPU architectures if you dig VERY deep down in the drivers etc. For everything else, you just need to run it through the compiler twice. I assume they have automated build systems for that. And for testing. If not, then that might explain some delays.

You have a valid point about the screen sizes though. Some UI bits have conditions based on the screen size. I would say ditch them and make it exactly the same but that's probably just me. So yes, the different screen sizes may add about 5% of the coding/testing effort.