You square the area when you double the linear dimension however when you are talking about three dimensional objects you have to cube it. A ball that has a diameter twice the diameter of another ball has eight times the volume. If you look at 20mm and 40mm projectiles, a 20mm shell is around a quarter of a pound and a 40mm is about 2 pounds, or eight times as much. It is not exact because the two are not exactly twice as big in all dimensions.
When the North Carolina was planned she was to have 14" guns. That was changed to 16" guns. Big deal you think, a lousy 2 inches. But if the shells were proportionate the bigger ones would weight 1.47 times as much as the smaller one.
The amount of paint needed between the two scales would be an extremely hard calculation due to a number of factors but we are talking surface area here, not volume so the square rule is more correct in this case. It is always better to have some left over than run out before finishing. Buy enough to cover the model, then add a little more. Get a large enough container and empty all the jars/bottles/tinlets together so the color doesn't change from one container to the next. Don't thin it until you are ready to use it. Some paints "turn" when stored after thinning.
You should have paid attention in math class.