What keeps Russian airliners from being as fuel efficient as their Western counterparts?
The engines.

Russian engines were for decades behind the state of the art. One reason was manufacturing tolerances. I remember that we studied the Progress D-27 three-spool turboprop and found that just the cast casing could be made 250 kg lighter with Western tolerances. The Russian designers kept their tolerances wider to make sure that the minimum wall thickness would not be undercut.

Also, since Russian designers had less powerful computers available to them, they could rely less on CFD, so the turbo machinery was less refined. Improved engines explain maybe two thirds of the increase in fuel efficiency over the last 70 years.

Another reason is system redundancy. Russian engineers made sure the aircraft would still be operable after being maintained by a substandard maintenance crew. Especially the military designs were built with the goal of maintenance-free operation for several weeks. This thinking was also applied on the civilian side, since all Russian aircraft were designed with dual-use in mind. In case of war, all Aeroflot planes would become military transporters, and Aeroflot crews received most of their training on military aircraft. Why else would designs like the Tupolev Tu-134 have a glazed nose?



Tu-134 (picture source)