I get the small window shrinkage as well. The app in particular is the USA one. After 1 or 2 episodes......it shrinks and continues that way until I back out and start the episode again. It will repeat all the time. I also use the ROKU Ultra and I can confirm it's not just mine alone.
I also purchased another ROKU Ultra and hooked it up at my friend's house and it does the exact same thing. Now, I know it's not the app itself, because I have a Sony Android TV and when I use that, it never had the small window shrinkage. I'm not sure what to try, but I'll go through the steps. Thanks in advance guys.