Perhaps this wiki on
mass will help, as it seems clear that it is being misunderstood. Mass has nothing to do with size or weight. Mass is measured in grams, pounds, tons etc. Size (and distance) is measured in meters, inches, light years etc. Weight is measured in Newtons.
The mass of planets can be determined through astrometry, the radial velocity method, and when conditions allow: pulsar timing. The radius of a planet can be found via the transit method.
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You're also not visualising this correctly, so perhaps you should have payed attention is math class, Havoc! Take a 2x2x2cm cube. It has a volume of 8cm^3. If 1cm^3 has a mass of 1 gram, then the cube's mass is 8g.
If you then double the size of the cube, it becomes 4x4x4cm, but the mass is not doubled because it is increasing in three dimensions, not one. So it now has a mass of 64g. That is a big increase for a mere doubling in size, an increase of 8, or 2^3. Remember, three dimensions!
A sphere is more complicated, though. As you all know , the volume of a sphere is described in the equation:
So scaling it up is more complicated than with a cube.
Besides, you can change the size and mass at a different rate, this alters the density of the object. The planet Saturn is less dense than water.