Actually weight is the issue as several reports showed, since the MK2 comes around 67t now in combatweight, even the MK1 is above 60t contrary to earlier aims of the development and it doesn't make sense to point to other tanks used by other forces around the world, since all that counts is the Indian threat enviroment and the operational requirements of IA. After the MK2 came out and the weight increase officially confirmed, we even have seen reports that the Arjun will have problems in certain areas of the western borders and therefor provides IA with operational problems. One reason why they might be more interested in FMBT, rather than Arjun MK2 in larger numbers.
That's the same problem that we see with LCA or AMCA, since these projects are "designed and developed" by DRDO, with the only imput of the forces being basic set of requirements. IA wasn't included in the design of Arjun and surely didn't demanded to make it wider and longer than the Leopard 2 tank, nor did IAF demanded LCA to be the smallest fighter in it's class, but both decisions turned out to be problems for the development. Arjun got too heavy and big, LCA got draggy and with too little interlan space for upgrades. So it's one thing that the forces providing basic requirements, but it's another thing what DRDO as the developer builds around these requirements, let alone if they are able to meet the requirements of the forces.
AMCA faces the same problem, IAF don't want it but still gave basic requirements for it and DRDO is developing a fighter now according to their own ideas with the aim to meet IAFs basic requirements (ignoring the fact that AMCA is operationally not needed). Unless we start to take the forces directly into the developments of these projects, by leading them and deciding about design and capability on their own, there will always be a difference between what DRDO wants to provide and what the forces actually needs.