RF efficiency of an oscillator (Pout vs Ic) depends by many factors, which are a bit different than those that affect the efficiency of an amplifier.
The output power of an oscillator is somehow inverse proportional to the open loop gain of the active device. Placing the active stage in linear Class-A (which is recommended for most of the oscillators) you get the highest stage gain but the output power is lower.
If you move the oscillator bias from the linear class (to AB, or B, or even class-C), the gain will decrease but the output power will increase. In this situation also the harmonics will increase, due to distortions that appear in the waveform. A LPF at the output will fix this issue.
The oscillator feedback also affect the output power. Various places around the oscillator stage could be used to pick-up the output signal, but each of them have pros and cons, in terms of output power, harmonics, impedance, influence on the oscillator parameters as frequency stability and phase noise..
An optimized simulation will give you the best oscillator performances.