Batteries don't have power, only capacity, but the capacity is in watt hours. Watt hours are derived from the battery voltage times the battery nominal capacity.
Your Kalkhoff battery is 26 volts 16 Ah, so that totals 416 watt hours (Wh).
The Bosch one is 37 volts 8 Ah which totals 296 watt hours, so the higher voltage reduces the apparent difference.
Any power difference results from the motor and controller combination and not battery capacity. Watt hours are like petrol in cars, more or less petrol in a tank doesn't make a car more or less powerful.