A AC motor can only be made so small and still work at 60HZ. PWM motors are a different story.
You need to use smaller wire, but length is important for the operating frequency. Less copper is lighter also.
You also have to dissipate the heat, and a smaller motor has less Mass/Surface to dissipate the heat it makes.
They do have better safety system to keep them from running hot, but they are not made to cycle continuous.
It is best to buy a pump with a good warranty, and hope the company is around to honer it, when it breaks.