Yeah that’s a little extreme right there, but you’re thought is correct. It’s all a basis of where the off-balance is located on the rotating assembly, and the total mass of the rotating assembly. 3 grams off near a main journal isn’t the same as 3 grams off on the outer diameter of the FW, nor is it the same between a rotating assembly of 100lbs or 40lbs 
On very high rpm engines I like to do internal balancing, then external balancing(FW, harmonic balancer, etc) that way if components were ever damaged and needed replacing, you can just dynamically balance the new parts and install them w/o having to tear down the entire engine to balance the bottom end as a whole.