But as far as I can remember,Tessi Thomas had on record told that DRDL was actively trying to cut down the weight of Agni III from 50 ton to ~22 ton or so.So cutting down the weight of Agni V should not be a problem either!!
Here is the link to that article:
Agni-V vital: Tessy Thomas - The Hindu
Besides,Agni IV has double the range and apogee of Agni II despite of having similar dimentions,displacements and warhead weight!!So I don't think that our propellant or rocket motor technology lags too far behind.