The first three areas are shown as square-shaped rectangles. Then at $1b, it switches to using a fixed height thus the 'size' is a linear dimension $amount/heig

The first three areas are shown as square-shaped rectangles. Then at $1b, it switches to using a fixed height thus the 'size' is a linear dimension $amount/height rather than sqrt($amount). Technically all these have the correct number of pixels, but we can perceive a larger magnitude range when laid out in squares rather than long rectangles with a limited/fixed height. There's nothing special about using a line, square, or cube to illustrate amounts. However if I were trying to have people actually get a sense of a large range I would use cubes as that's the highest dimension we can intuitively grasp. e.g. picking up a metal object of visible volume weighs about as much as we expect.