Ok the best way to explain it is the higher the watts the higher the capacity of the charger to charge at 6amps on batteries over 5900mah.
So lets say you have a 6000mah 2s battery and a 6amp 50w charger divide watts by volts.
50/8.4=5.95 amps max. so, a 50W charger will almost charge at 6000mAh,a 100watt charger will charge batteries over 5900mah at 6amps.
Hope that helps Ron.
Cheers Guru
The mAh rating has nothing to do with the peak power calculations.