There are two answers to your question, yes and no. A lot depends upon the capacity rating of the ESC and the current draw of the motor under load. Ideally, the ESC should be over capacity. It should be able to handle any current draw from the motor if the motor were to stall, ie, wrap in weeds, run into a rock, or if you were to lock the shaft, ect. The ESC should also be protected by a fusable link to protect it and the motor.
Back to your question, if your ESC has a capacity of say 40 amps, and your motor draws about ten amps at stall, then yes you could use smaller wire. If your motor draw were say 25 amps, then I, personally, would use the same size wire as the ESC. Manufacturers are notorious for rating ESC's according to their standards, not a universal one. Better to be safe.
Lastly, wire size is not the only factor in a wire's capacity. The number and size of the conductors that make up the wire are critical to it's capacity.
I you err on the safe side you will be far less likely to have problems in the future.