The key is for a given output. A cone can cycle at 40 hz, but the distance between stops (i.e. Where derivative is 0) will vary depending on the amplitude of the signal. A smaller cone has to travel further to move the same amount of air as a larger driver. So a large cone might move 5 mm back and forth to produce a given output at a given frequency. But a smaller cone might have to move 7.5 mm. And do so in the same period of time to keep the signal at 40hz.
If im wrong, show me the maths! :)
If im wrong, show me the maths! :)