Remember, it is the moment of inertia of the blade or disk that matters, rather than its weight alone. The MoI depends on the square of the length (bar blade) or diameter (blade plate), so a bar blade has quite a lot of inertia effect. It seems to me that the blade disks on most Australian mowers I've seen have considerably less mass and MoI than a Honda bar blade, but genuine Honda blade plates are remarkably heavy too, like their bar blades.
I believe on mowers the weight of the blade disk is made sufficient to prevent kickback. Some manufacturers probably have larger safety margins on this than others, but generally, when the mower leaves the factory it is unlikely to kick unless the operator uses some odd starting technique.
Ty, I'm told that people manage to bend crankshafts by hitting things, whether they have a bar blade or a blade disk. It seems that some of them don't give up easily after they hit the rock, they try to machine it away to a flat surface, with the mower.
