What would cause huge sparks when jumping a battery?

A full charged battery has a higher voltage than a discharged battery and the voltage will travel from a higher to a lower voltage point. In dry air, voltage will "jump" at 10,000 volts per inch. Thus when you get close to the second battery, within a fraction of an inch, you will receive a spark. Notice to receive this spark you already had one side of both batteries already connected. The best is to connect the negative sides of both batteries first as should your jumper cable become too close to a spark plug wire it will most likely go to the frame of the car and not you! Spark plugs are typically 18,000 volts!