CDRW
Try this simple experiment.
Place 10kg of groceries in a shopping bag. Stand up, place the shopping bag on your shoulder steadying it with your left hand. Fully extend your right arm in front of you and raise it to fascist salute level. Easy wasn't it?
Now move the shopping bag to the fingers of your right hand and repeat the excercise. It is more difficult and more strength is required.
Your legs can easily cope with the weight, but you are struggling to rotate your arm about your shoulder.
If you imagine that your shoulder muscles are the elevator and that your right hand is the nose of the aircraft and you will see the problem.
A 3 tonne error in TOW is minor, but 3 tonnes assumed to be in tail, then placed in the nose is major. It is not about weight, it is about trim. On some a/c trim limits become very restrictive at high weights.