The weight of the object depends on gravitation field g and mass of the object m. i.e. weight =m*g

And Usually the mass/size of the object is much smaller than the planet.

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Also, I would like to be able to change weight of the planet and not only the radius.
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Do you mean the mass of the planet instead of weight of the planet?

The mass of the planet is proportional to volume of the planet. M=\rho\frac{4\pi r^3}{3}