They're actually converting AC to DC for electronic devices, so there's a lower amp load than most AC plugs.

They are larger, because the converter which converts alternating current (AC) to direct current (DC) is connected at that location, so the converter doesn't hang from less durable wiring (wire strain), or become a tripping hazard, etc.

The power supply on many newer laptops have a typical ac plug-in at the outlet end, and place the converter somewhere in the middle of the power supply cable.