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.

Also, I would like to be able to change weight of the planet and not only the radius.
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}