A horse is as light or heavy as the rider makes it. Horse ridden in the biggest, fattest snaffle known make for a much harder mouth than a more severe bit ridden with feather soft hands.
Dressage is supposed to be harmony between the horse and rider. A subtle communication that is all but invisible. If you have to have your horse feel like 50 tons in your hands, than that is not light and invisible and certainly not an example of self-carriage.
If your horse needs that much help between hands and mouth through piaffe, one tempi's and canter pirouettes, then you need to go back and work on self-carriage, engagement behind, and lightness. Maybe say first level/second level work. And that, my dear, is the problem with dressage today.