Yeah, the amount of propellant can be smaller, provided the impulse is caused by the heat from the nuclear reaction. That could greatly cut the amount of propellant needed, which of course is reduces the overall weight, making the propellant used even more effective... Asimov's fictional rockets (the more or less conventional ones) used nuke-powered steam engines, and his explanation was that the mass*velocity of the propellant almost equals the mass*velocity of the craft (in any design using propellant), so less propellant going faster is the same as more propellant going slower. :')
That's right. Less fuel is needed, and then less weight needs to be lifted because there is less fuel. The multiplier effect.