It's not just the unsprung weight. It's the mass moment of inertia of the wheel/tire. The farther you push the weight outwards, the greater the wheel/tire's mass moment of inertia. The equation of rotational motion is torque=inertia*angular acceleration. The mass moment of inertia for a cylindrical object is 0.5*mass*radius^2. So if you choose a 17" wheel over a 15", in the inertia equation you've changed the (15/2)^2 to a (17/2)^2. You'd have to choose a 17" wheel with a mass about 0.779 times mass of the original wheel to keep inertia the same.

Note that this doesn't include that you'd have to also calculate the change in inertia for the tire. You would use the inertia formula for a thin ring which is mass*inner radius^2. So using a 17" tire, you've increased the mass moment of of inertia of the wheel/tire combo again which means you'll need an even lighter wheel.

Clear as mud?