I saw this video and thought it was fake, but I wanted to be sure. I did some googling and found out that it is called a Jastrow Illusion. I still didn't think it was real so I found this similar optical illusion:
I still didn't believe they were the same size, so I copied and pasted it into Microsoft Paint and cut out the bottom image and dragged it ontop of the top image. They turned out to be the exact same size, and I was a little amazed.
From: Random Good Stuff
and Might Optical Illusions