How big is the aquarium? -- a one gallon generator is pretty large.
You can reduce the volume of the generator, go to a 2 liter bottle.
You can use less yeast and /or sugar.
You can make the reactor or diffuser less efficient, maybe, depending on what you are using.
You can blow off more CO2 by increasing surface agitation, aim a spray bar towards the surface or lower water level to allow a HOB to waterfall a bit.