I came here to point out exactly this: If you only shrink the ball, without reducing its size, well… you’re gonna have problems carrying the ammo.
As a DM, I think I would let them both shrink and reduce the mass, and wait till they fired the weapon before invoking “conservation of momentum” and declaring that the cannon ball drops to the ground after about a meter.
Exactly this. I think it’s important to make a point out of checking that they’re not injured, but without exaggerating the situation. If we adults get stressed out, they’re more likely to get stressed/scared as well, even though they’re not injured and not even really in pain.