What is the spark, caused by opening/closing a circuit, dependent on?
Sparking is normal.  Every switch will do this.  It's why switches wear out.  

You don't need much current to get a spark.  Ever get a static-charge zap after walking across a carpet and touching a doorknob??  That's nano or picoamperes.  (At thousands of volts...)

The amount of spark, and the resulting "damage" to the switch contacts, is a function of both voltage and current.  i.e. "power".    

There's also a component of inductance.  If you cut the power to a spinning motor, you get a huge kickback voltage and a big spark.  THis is why (non-electronic-controlled) power tools have a switch that goes "click" when you use it... It's designed to make and break that contact very very fast.