Yes, aspect ratio is the key, not the actual pixel size:
600:450 = 1.33:1
533:400 = 1.33:1
900:675 = 1.33:1
I experimented and all the above cover sizes are a nice fit for Marvin. Even the jumbo 900:675 cover fits the same way as the smaller covers. For some reason,
600:400 = 1.5 has fallen out of favor with Marvin and/or iOS-9.
So, the value of this revelation ... and $2.00 ... will buy us a lukewarm cup of coffee!
Have a nice day.