CFM56 heavier than the J57? I'd have to see the numbers.
But take that at face value; the mission fuel is considerably less, and with more thrust, the climb is considerably improved, reaching TOC sooner and higher.
This also translates to longer endurance and range, and/or more donor fuel to their "customers". (If USAF had put the bigger 707-320 wing on it, the advantage would have been even greater)