I agree. Not sure what makes the balls so heavy (lmao) I think that they do damage because the weight is evenly distributed completely throughout the ball, which would nearly be the same for Bonsly, with the exception of its Bonsai branches.
Btw, the equation looks like this. Force = Mass X Acceleration. Lets say, just for scientific purposes, that in Brawl Bonsly is thrown at 10 mph (even though we know he prolly goes faster) The equation reads, Force = Mass(38) X Acceleration (10).
The ending result equates to Force = 380 lbs. Note that this is not taking into account Bonsly's size.
So basically, Bonsly, a rock, comes flyin at you with 380 pounds of Force. Your ass is dead.