The engine driven fuel pump forces more fuel through the mixer on higher power settings. Otherwise your engine would be deliberatly flooded when idling on the ground with the mixture set to rich. The mixture only controls a ratio ( in ideal conditions).
The engine would only tend to detonation on very high power settings with a too lean mixture ( t/o). You won't get any engine to detonation on a 50% ( or even higher) power setting whatever the mixture is.