Building BLDC ESC - question on PWM duty-cycle vs motor speed
A BLDC motor is similar to other motors in that the torque produced is proportional to the current, and the speed is proportional to the applied voltage (assuming commutation is done correctly.)  Of course there are second order effects, but that's the basic idea.

So if your motor was ideal and lossless, running at high speed with no load would be zero power output and would require no current.  With a real motor windage and other losses increase with speed even at no load, so there is more current required to overcome those losses.  

The first-order increase in speed comes from an increase in effective applied voltage, which  comes from appropriate commutation and PWM duty cycle.