The chargers (Home/110 V) charger for the iPad is physically larger that the one for the iPhone. I assume the former is of a higher capacity. That said, is there any harm in using the one designed for the iPad on the iPhone? The one designed for the iPhone on the iPad?
The iPhone charger may not charge the iPad (or charge it very slowly), but you can’t hurt either device by swapping chargers.
The iPad charger is 10W. The iPhone one is 5W. Both will work without damaging the other, but it might take a whole day to charge the iPad on the 5W wall wart.