The principal issue appears to be broadcast power. The smallest one linked below (weighing ~400g without a power supply) has a burst power output of 130W (compa

The principal issue appears to be broadcast power. The smallest one linked below (weighing ~400g without a power supply) has a burst power output of 130W (compared to a cellphone which might have an output power of ~2W). The actual amplifier itself wouldn't be huge (although heat dissapation could be), but there's a lot of capacitors and power supply circuitry needed to deal with that sort of power output. The size of all the components basically depends on what the duty ratio is - if it's a few uS every second, then it would be much easier to shrink than if it has to broadcast ten or hundreds of times a second.