How does this work?

Each of our eyes has six muscles attached, these muscles work in pairs, with one that pushes and one which pulls the eye back. As you stare at the centre of the disc the muscles which are pulling your eyes to focus on the disc become tired. When you look towards your hand the fresher muscles pushing the eye back into its relaxed position push too far, this causes you brain to become confused and process a new image, this gives the illusion of growing or shrinking.