Yeah, that's why I mentioned low tongue weight and why you typically won't find an extender rated the same as the actual hitch. The manufacturer of the extender, if any good, is taking that into account with their rating just like when a hitch is rated for a specific WC amount they are assuming within a certain set of parameters because they don't supply the draw bar or hitch haul or whatever.....but luckily an 18" extender cannot turn 400lbs into 1000lbs.

 

Extenders, at a minimum, double the length between the ball and cross bar of most receivers. That's a doubling of the the torque on that bar.