It appears to me that the lighting level changes four times in that video.
There aren’t any lighting changes, it’s just a really crappy camera.
It is a very simple illusion. The pieces are a curved shape and so the outer arc is much longer than the inner one. (Think concentric tracks on a racetrack.) So if you place them next to each other, outside next to inside, they appear to be different sizes. The shallow angle of the camera adds to the effect.
Here (scroll down to #9) is an example of the same effect.
ETA: Here (scroll down to #11) is another example of the same thing.
The photography is also playing a big part.
A wide angle lens at close distances will exaggerate the apparent size near objects compared to almost near objects. If you look his hand in the back of the scene it is a good deal smaller than his hand in the front.
This has been a magic trick that magicians have used for years. It is definitely an optical illusion as friedo described.
You never had Brio train sets, did you?
That’s one of the oldest optical illusions going. It’s real and because they are arched, it makes them look that way.
Look at the corners of the top edges and the corners of the smaller, close edges. The corners line up vertically. That’s where the optical ‘trickery’ shines through (pun intended).
I couldn’t see the optical illusion. It just looked like two arches being moved around.
The illusion is well-known and authentic. But at first I thought this particular presentation could be faked because the colored piece is moved out of frame, providing an opportunity to switch it. However, its shadow seems to confirm it wasn’t.
That’s what I was thinking. I couldn’t figure out what they thought they were showing.