Bundling three or five first stage cores together improves the launch capacity a great deal. But Delta, Falcon and Angara are supposed to use the same upper stage in their multiple common core versions, as their weaker single core versions do. They double or multiply the amount of mass they can put in orbit, so why aren't larger upper stages used for the common core versions of those rockets?
For example, if a Falcon 9 launches 20 tons of fuel to dock in LEO, how can the same upper stage be used to launch 40 tons of fuel to the same orbit? Doesn't it need to be larger and maybe sturdier to support the much heavier payload?
Similar with Atlas V that has a pretty wide range of capabilities using zero up to five solid boosters. But it always uses the same single engine Centaur upper stage.