Can someone explain to me why my bluetooth speaker (running off battery) produces a more powerful magnetic field than my 2-piece USB powered speakers (powered by a power bank).

The bluetooth speaker has 2 x 1.5" drivers and 1 passive bass radiator, whatever that means. 12 watts RMS.
The USB speakers have 2 x 2" drivers and 2 passive bass radiators. 4 watt RMS.

I'm playing them at similar, low-mid volume.
The electric fields they produce are about the same.
The bluetooth speaker is producing a little bit of RF but that part is obvious why (the bluetooth signals).
But there's quite a big difference in the magnetic fields. Measuring right up against the units the bluetooth speaker is producing a field of average of ~50 mG, spiking over 100 mG every 2 or 3 seconds.
By contrast, the USB speakers never go above ~20 mG, and no spikes either.

Any clue? I'm trying to learn about EMF. It's very interesting.