Not obvious
Why is the obviously smaller Jupiter capable of launching a much heavier package? Are we comparing a lightly-developed theoretical rocket that "ought" to have this lift capacity with a more-developed rocket where they've learned the hard way that the performance and weigh margins aren't as good as they thought on day 1.
I bet Ares as-originally-conceived was supposed to have much greater lift capacity.
Vapourware is made of really light stuff, so it flies well. Real vehicles are made of metal and engineering and backups and safety factors. That stuff is heavy...