Indeed, according to most of catalogues the efficiency and power factor decreases if the rated power decreases [and also if the rated velocity decrease].The slip increases. The weight per power unit increases.
In my opinion this could be the explanation:
The motor dimensions-the stator inside diameter at the air gap- is stated according to motor
rated torque.
D=K*T^(1/3)
For the same rated rpm the torque increases proportionally with the rated power.
T=K*P/rpm then:
Dint=K*P^(1/3)
Actually Weight1/Weight2~=(P1/P2)^(a/3) a=1 to 2.5
That means the weight/hp is less for high hp and then mechanical losses[friction, ventilation] and
magnetic losses are less for more power rated. Also the slip decreases if power increases, so the electrical losses Plosses=Pm*(1-s) will decrease.
