Scaling problems with a cake - mousse layer increases more than other layers
Density = mass / volume, so if you double the mass of mousse, the volume should also double. The only way you can get more than double volume is if the density decreases, which I doubt is happening.

If you're covering the sides and top with mousse, than mass--which is directly related to volume--isn't going to help, you need to think about surface area. For a cylindrical cake frosted on the sides and top, the surface area and volume are

SA = pi d^2 / 4 + pi d h
 V = pi h d^2 / 4


with d the diameter and h the height. You could imagine two cakes of the same mass, one thin and very broad with a very big surface area, the other a bit tall and small around with much smaller surface area. So you'll need to take both height and diameter into account when figuring how much more mousse you need.

Edit

We can simplify a bit. The ratio of new mousse to old mousse in volume or mass should be the same as the ratio of new SA to old SA. Using your edit that the height stays the same:

Ratio = SA_new / SA_old = d_new / d_old * (d_new  + 4h) / (d_old + 4h)


I'm going to assume that your height is pretty big, and that four times the height is maybe getting close to the bigger diameter. Then we could approximate the ratio above (there is some height h such that 4h is between d_old and d_new such that)

Ratio = d_new^2 / d_old^2


So if you're going from an 8-inch diameter to a 12-inch diameter you'd want to make 144 / 64  = 2.25 times the mousse. And, as this is pretty rough, I'd add a little to that to be safe and make 2.5 times the mousse. (And having a little extra mousse doesn't seem like the worst thing in world.) If my guess about the height is wrong, just measure and use the actual formula, you'll get better results! Always make a little more than the you expect to need though, because nothing's worse than having not quite enough.