I sort of see it too. I think it's caused by a mix of 2 things, first when you stare at something for a while the details tend to fade away, and this image is

I sort of see it too. I think it's caused by a mix of 2 things, first when you stare at something for a while the details tend to fade away, and this image is very susceptible to it since it has very soft colors on the border, and the gradients are from the outside towards the inside so that causes the perceived object to shrink. Second, the eye constantly has small involuntary saccadic movements, and whenever that happens the first effect gets "reset", and the perceived image grows to its real size again.