I was reading a NASA study talking about the benefits of flying the Dragon spacecraft vs the Space Shuttle. The study mentions that both launch systems use only about 2/3 of their payload capacity when delivering supplies to ISS. Given that extra payload shouldn't cost any more, given that the rocket is the real cost, not the fuel, I don't understand why cargo wouldn't be maxed out?
My first thought is that, by reducing the mass, it makes it easier to maneuver and adds some extra margin of error? But it seems like, if that margin of error is necessary, shouldn't the "safe" payload be the official figure?
My other thought is that maybe the height of ISS means that the Dragon needs a smaller payload to make it there?
The problem with these two explanations is that, since the Dragon at least was designed to just go to the ISS, then you'd think that the maximum-designed payload would be the normal amount of stuff they would send?
If anyone knows the answer it would be much appreciated. Thanks!