Steve got it.
The film was done on a very small budget and they were running out of money, so they created a flat cardboard airplane on a smaller scale (I guess renting one was too expensive). This worked, but they needed smaller scale people to match the plane. The whole reason there is so much fog in that scene is to cover all this up, but once you know, it is incredibly obvious. It doesn't detract from the movie though. Its still the greatest film ever made.