Gain compression on an Ids vs Vds curve in Power Amplifiers
You just have to know what to search for .

https://www.microwaves101.com/encyclopedias/compression-point

The small signal gain changes with bias voltage in FETs and bias current in BJT's. Thus the large signal swing (not obvious on your hand drawn chart) changes more near the threshold of turning off or near clipping.  What happens is the signal has more gain on one polarity than the other. This results in THD on the signal as well as in power amps the gain is compressed below projected output.