There are no easy answers to efficiency questions. It may appear easy but it’s not as measuring the overall combined efficiency of a vfd, motor and pump, known as watts to water, is problematic at best. Some incredibly pricy and fussy gear is needed to accurately measure the power in and out of a vfd. Measuring motor power is easy but it requires a dynometer, anything less is an estimate. Pumps appear easy as you can measure pressures and flows but still not have all the data since you’re missing the delta T of the water.
It’s not that a smaller drive will be more efficient, it’s just that the measurements with a drive that matches the motor rating will have better resolution, nothing more. If a smaller, quieter drive is a better fit for you, go for it. Just don’t expect any performance improvements.