It was huge capacity, but at a higher cost, so you try to get your payload to fit on the cheapest rocket you can.
Starship turns everything upside down and, suddenly, makes the cheapest rocket a Saturn-V class heavy booster. With it, it's cheaper to add a huge kick stage to your Neptune probe to make it get there faster, put more solar panels so you don't need to deal with compliance around an RTG, or just use steel for structural elements (because why not?), and so on.
Starship turns everything upside down and, suddenly, makes the cheapest rocket a Saturn-V class heavy booster. With it, it's cheaper to add a huge kick stage to your Neptune probe to make it get there faster, put more solar panels so you don't need to deal with compliance around an RTG, or just use steel for structural elements (because why not?), and so on.