Ok bigger carbs do not provide more fuel they provide more air so you need bigger carbs once the ones you are using cant meet the engine demand in air.In essence you dont want to choke your engine by using carburators that cant provide the air needed for your engine to reach its full potential.On the other hand if you over carb your engine,you kill the velocity at wich the air is drawn into your engine, not much better...
When do you benefit from bigger carbs? Size of the engine,porting specs,pipes,the type of riding/racing you plan on doing, etc. will pretty much dictate if you will benefit or not from bigger carbs.