The current delivered into the device may well be less. Voltage is regulated down to the point that the port shuts-down. So assuming you don't have it on some uber-long extension, you should get pretty close to 5vDC. But amperage is another story. And that's the umph you need to heat the thing up.
First, we need to remember that Volts * Amps = Watts, so Watts / Volts = Amps.
Now, think of a hair dryer, or an electric heater, or a curling iron or (man, hard not to come up with the girly analogies...)
Take two hair dryers that both run from the 120v house current. The 1800 watt hair dryer gets far hotter than the 1000 watt hair dryer, which gets hotter than the little 750 watt travel hair dryer. They're all running from the identical 120v circuit.
1800 / 120 = 15A (the max you should plug into a typical house plug here in the US)
1000 / 120 = 8.33A - just over half the _power_
750 / 120 = 6.25A - a little more than a third the _power_.
If the USB can't deliver adequate power, three things can happen. If it's close, then the device will run poorly. If it's a long way off, then either the protection circuit on the motherboard (kinda like a resettable fuse) will shut down power and the port will appear dead until it cools down enough to reset the fuse, or on an old and cheap motherboard, the chipset delivering the USB power will burn-up and that port will be dead forever.
(Wow - and with this post, that's 1000 cigarettes I haven't smoked! Amazing how that math works!)
