The absolute truth of this puzzle is simple: Shapes 'A' and 'B' are completely identical in size and area. * The illusion is entirely created by how the two identical curved segments are stacked and offset.
When placed like this, the short inner radius (the bottom edge) of shape 'A' sits directly next to the long outer radius (the top edge) of shape 'B'.
Because our brains naturally compare the two edges that are touching rather than viewing each shape as a whole, the contrast makes shape 'A' look shorter and compressed, while shape 'B' appears stretched and elongated. If you were to pick up shape 'A' and slide it directly on top of 'B', they would match perfectly.
