Taken from the Box2D Manual:
The fixture density is used to compute the mass properties of the parent body. The density can be zero or positive. You should generally use similar densities for all your fixtures. This will improve stacking stability.
The mass of a body is not adjusted when you set the density. You must call ResetMassData for this to occur.
fixture->SetDensity(5.0f);
body->ResetMassData();
You may need to Reset the mass data in order for it to work properly.