I see here some confusions.
Both T-90 and Arjun are in the same tank classification level, both are Main Battle Tanks. Please, accept reality, nor weight nor size is responsible these days for vehicle protection, it was maybe during WWII and directly after to the 60's, but not any more.
Why smoothbore guns will do better, it is not only about HE ammunition. Smoothbore have also longer service life, and can fire much powerfull APFSDS ammunition, however we newest, and more powerfull smoothbores have problems, did anyone of You know what will happen with gun barrel when You will fire several times such powerfull ammunition like DM53 or M829A3?
WARNINGS! In combat emergency conditions, when the lives of the crew are in immediate jeopardy, there is a low but real risk of gun tube failure if the M829A3 is fired between 120F and 135F. If the round is fired under these extreme temperature conditions, crews must examine the tube for unusual wear or other abnormalities after firing 6 to 8 rounds,and must have direct-support maintenance personnel inspect the tube up ti 18 rounds are fired. At propellant temperature above 135F, the risk of a catastrophic failure of both the round and the gun rises significantly.
Before firing M829A3, check the tube serial number listing. NOTE. If firing tank does not yet have M829A3 subdes, this round can be fired using M829A1 solution(sabot subdes 5 for M1A1 AND "M829A1" for M1A2/M1A2 SEP)
This will happen, and taking in to account India's climate and temperatures, it might be possible that sooner or later IA will need to buy smoothbore gun for Arjun tanks if they want more powerfull APFSDS, because rifled gun might not withstand, not even a pressure but simple wear and tear, rifling do not like sabots etc.
As for smoothbores, HESH etc. But smoothbores have analog to HESH, it is simple, non programmable HE, HEF and HEOR ammunition, that's all, and to make them even more deadly, someone get an idea to use programmable ammunition. Hey look in NATO even IFV's should have HE or HEI programmable ammunition ranging from 30mm to 40mm (for example 3P ammunition designed in Sweden for CV9040 IFV's).
Rifled gun will never have such capability with HESH, because HESH do not have any perspectives in long term, it is allready dead.
Think about these problems, think in long perspective. India can import such ammunition, look inside and later design it's own programmable ammunition. But nooo, some people seems to think that better is reinventing the wheel each time...
Arjun R&D phase is also funny, no seriously it is funny, it was first designed as 40 tons tank, later when there was possibility that Pakistan will have M1, they changed specification of vehicle. Then M1 threat disappeard, but it seems that IA seen the only tank capable to stand against M1 in Germany, yeah Leopard 2, so they probably asked Germans to help design their new tank, but obviously Arjun not only have some weak points of Leo2 design, but adds few more... besides of course many good sides of design, like suspension, more care to make ammunition storage in hull a bit safer etc.
But to me it all seems too fishy, and Mk2 from showed image, don't look as a deep modernization, but it's maybe me.
Please don't get me wrong, I just think loudly why R&D phase go that way not another...