Because the field lines are closer together the smaller the plasmoid. Imagine the gravitational field around a planet. If you took the same planet (same mass) and shrunk it down, the surface gravity would be greater, because you’d be standing closer to the center of mass.
So the size of the plasmoid depend of the distance between the anode and cathode? And even before short distance between them would get the lines of ions and electrons which carry the current closer and create a stronger magnetic field. That is if the current still are the same. So far so good, but doesn’t a small cathode and its smaller anodes feels more stress for the same current?