Assuming the same wheel manufacturer, and even if the two wheels were the same weight, the moment arm of the larger wheel will be longer because of the rim's construction at the outermost end. In other words, it's not the spokes that are heavy. Its the "rim of the rim" and the drop center that weighs a lot.
Tires are heavy so using a lower profile tire may save more weight than using a smaller rim, but if you compare equal profile tires (again, same for same) then I don't see how the larger rim could possibly have an advantage. The larger rim has the drop center further from the hub, and the tire has its mass further from the hub as well.
Contrast that to the lower cg that one gets from running a smaller tire and resultant lower amount of weight transfer, and I think the advantages of a smaller rim/tire combo are complete. Less weight, smaller moment arm, lower cg, less weight transfer. The cost of all of these benefits is the smaller (shorter) tread patch, and the limitation as to the size of brakes that will fit inside the smaller wheel. You see the same discussions with flywheels, people talk about a heavier flywheel acting the same as a lighter one because the weight is removed from the "inertia ring". John also points out that the rim and tire choice makes a huge difference as well, so that makes this a theoretical and not so much a practical discussion.