Other issues are thrust differences for each phase. Booster thrust is greater to get the vehicle initially moving against gravity. Then come the lesser sustainer thrust to keep the vehicle stable in both heading and velocity while the Earth rotate below. If India's targeting criteria does not involve tens of thousands of kms, then booster thrust to get the missile moving quickly is all that is needed.
There are many issues involved that have nothing to do with technology but affect the final distance. But again, I would go with targeting criteria before technology.
You are asking a loaded question. But the gist of it have to do with propulsion and guidance technologies. You should understand that we live on a rotating sphere. If the missile is launched high enough and therefore stay aloft long enough, the Earth's rotation will move the target below the missile, this is assuming a launch against the Earth's rotation. Launching with the Earth's rotation require a slightly different ballistic arc and speed. The Earth's rotation, time of flight, and time of free fall, are what give the sort of illusion of distance.
Target location also matter. If India's targeting criteria does not require tens of thousands of kms, then what need is there to have a propulsion system that can reach that far? Lighter weight can mean more sophisticated and smaller nuclear warheads and that can contribute to the distance differences.