A planet size graphic is useful only when it explains what size means. SizeGuess compares a stated diameter. It does not display the distance between planets or the scale of their orbits.
Diameter and radius differ by a factor of two
A radius runs from the centre to the edge. A diameter runs all the way across. Doubling a radius produces a diameter, so comparing one planet’s radius with another’s diameter would create a misleading result.
Equatorial and mean diameters differ
The eight planets in the Space collection use the equatorial figures in NASA’s planet-size guide. A rotating planet may be wider at its equator than between its poles. A mean-diameter table uses another convention; it can be accurate without giving the same number.
The Jupiter and Earth comparison uses both equatorial widths. The Saturn and Jupiter comparison excludes Saturn’s rings, because the selected measurement concerns the planet’s body.
Width ratios do not equal volume ratios
For ideal spheres, doubling diameter multiplies surface area by four and volume by eight. The game’s displayed ratio describes a linear dimension. It should not be used directly as the ratio of mass, area or capacity.
A shared scale can make a small world hard to see
When one object is much wider, the smaller one may become tiny on a phone screen. The comparison tool preserves the scale rather than enlarging it separately. Use an intermediate object, such as a large moon, to make the progression easier to inspect.
Try Earth and Mars, then Earth and the Moon, before playing the planet size guessing game.