The reason the 540 turned the prop slower than the 380 was because the 540 has a lower Kv rating.

Electric motors are not like glow engines. If you put a smaller prop on a glow engine, the engine spins faster until the torque that it takes to turn the prop is equal to the torque the engine can make.
Electric motors spin at nearly the same rpm with a big or little prop. There is some speed sag with bigger props but it is not nearly as drastic as with engines. Put a smaller prop on an electric motor and instead of apinning faster, it just draws fewer amps from the battery. Even a shaft run without a prop has the motor turning only a few hundred rpm faster than it would with flying prop. By contrast, if a glow engine throws a prop and goes into a shaft run, it can rev high enough to self destruct.

The rpm that the motor tries to turn is determined by the motor's Kv rating and the battery's volts. Kv is given in rpm/volt or rpm per volt.
If a motor has a Kv of 1000rpm/volt, then giving the motor one volt will result in 1000 rpm shaft speed, two volts gives 2000 rpm, 10 volts gives 10,000 rpm etc.

The reason that the bigger motor was a complete dog in the airplane was because its lower Kv meant you needed more cells to get to the same rpm. Or a much bigger prop to load that motor at the lower rpm.

What do the numbers on the motor cans mean? I have no idea. There is no consistancy from maker to maker. They might as well just give them names. You have to look up the specifications. In addition to Kv, there is also maximum amps, internal resistance, and maximum cell count.