It's sort of a battery, you just can't charge it with electricity. If you oversize the generators, you then can discharge the battery when renewable generation is low, and let it charge when renewable generation is high.
Increasing capacity at hydro power stations or other baseload stations isn't as easy as just "putting a larger generator in". Everything needs to be considered and sized up: turbines, piping, outlets, new environmental assessments, etc., just as in any other type of power plant.