Apple TV zoom/stretch (Netflix app)
Double-tap + Swipe

Quote from this Reddit thread (emphasis mine):


  I've been running into this accidentally and thought it was a bug: 4:3
  videos would zoom up (proportionately) to 16:9 from time to time while
  working the remote. It turns out there's an (AFAIK) undocumented
  gesture: double-tap + swipe left (or right). So, whenever you're
  viewing 4:3 content, you can tap, tap and swipe left to zoom up to
  16:9 and repeat the gesture to go back to 4:3. Neat &amp; hope it helps
  someone. There's quite a bit of old conference talks in 4:3 out there.


I can't explain it any better than that, but hopefully it helps.