The mass parameter in the txt file only affects the main body in the gmf. Simple components like spikes and ram plates only have one part so there's no problem.
If the component has multiple parts like a motor or a wheel, then the weight of the axle, traction zone, etc. gets added on top of whatever the main body's mass is. So you have to subtract the weight of any extra objects in order to get the desired weight for the main body.