About Radius and Surface Area
You seem to be confusing the simple mathematical relationship between radius and surface area, and the more complex relationship between mass and size.

If you double the radius of a sphere the surface area quadruples.  This is pure maths, and is not particular to stars. The volume is multiplied by 8.

But in a star, increasing the amount of matter by a factor of 8 will not result in the volume increasing by a factor of 8. The relationship between the mass of a star and it's volume is not simple and depends on factors like age, and the particular combination of elements in the star.

Doubling the radius does quadruple the surface area.

Doubling, or even increasing 8 fold, the mass does not quadruple the surface area.