Measuring a couple of images on Google it looks like the two smaller ones total 1.5X wide and the larger is 2.4X. That happens to match up with the Golden Ratio, so it's quite likely that's what they started with.
If you apply the Golden Ratio to the 1.5X, then the two smaller ones would be 0.6x and 0.9x Measuring on my monitor is super inaccurate, but that sure seems to be what they are.
If you look around on Amazon and eBay there people are making them in all sorts of ratios and total widths, so what do I know?