On my 790, one can ad 6 (42 lb) weights. This comes to a bit over 250 lbs, which is far less then the 419 loader weighs.
However, it's more a matter of weight distribution rather then just bulk weight. The point is to keep the front end down for steering control, not totally counter balancing a 600+ lb rotary cutter.
I've no idea if this is a typical weight (250#), but I expect most CUT's will use between 200 and 300 lbs of front weights.