Why does curling a wire decrease its amperage/wattage?
The answer is "heat".

When an electrical cord is in free air, heat generated by power wasted in the cord's resistance is able to dissipate, keeping the temperature relatively lower. When the cord is spooled up, by contrast, the heat accumulates and the temperature rises higher, for the same level of wasted power.

I don't know if there is a formula for this, or if it is modelled in simulation, or if it's just measured.