Movie set set-up #1: The round set has the meditation prop wheeled into the center of it to get the shot of Vader walking out and kneeling on the pad. He is walking from the center of the set to the outer edge of the set, as indicated by the brown/gray ring in the lower photo.

Movie set set-up #2: The set isn't big enough to house the room for the meditation chamber prop and the area where the Emperor effect will be projected, so it is rolled out of the set and ol' Dave Prowse turns around on the pad to now face into the set to stare into ol' Chimp-eyed old lady.

You cats are thinking way too much about this. This is just an example of 'movie magic' where film makers have to make do with small sets in order to make them look much larger.