But it's Not smaller
I think that the obviously smaller Jupiter is you comparing the ARES 5 to the Jup-120, which is not the payload comparison touted by the J folks.
J proponents tout their starter vehicle J-120's much higher payload to LEO vs the ARES 1 'Stick'. The Stick has the POGO and weight problem. ARES 1 is a stretch SRB (low ISP) with a new 2nd stage. J proponents say that their J-120 entry class vehicle is superior and that there is time to develop the bigger J-232 for when the 2020+ moon missions are due.
They're NOT claiming that the A5 has inferior performance to the J-232. Or I missed it.