Strictly speaking you are correct in that the diminsons of the rotor system are larger. The MI-6 does have a larger rotor diameter but it is equipped with 6 blades rather than 8 as found on the Mi-26, also it cannot lift as much externally and the internal payload capacity is less. The Mi-26 will has significantly more room internally than the Mi-6 as well.
Also, consider the shaft horsepower available for each engine (I suspect, like most helicopters both aircraft are limited by the installed transmissions/gearboxes). When you conisder that the Avco-Lycoming T-55 series of engines on CH-47Cs were rated at 3350 SHP and the engines on CH-47D were rated at 3750, even the CH-53E, with 3 engines rated at 4380 SHP each, is still puny in comparison towhat the Russians have developed.
Mil Mi-6 Hook
This big transport, the first turbine powered helo of the Soviet Union, was not only for a long time the largest helicopter of the world, it was also the fastest. More than 800 units of this model, which can carry up to 70 troops, were built
Rotor diameter: 35 m
Length: 41 m
Height: 9.8 m
Weight: 27240 Kg. - Max: 42500
Engine: 2 Soloviev D-25V of 5500 hp each
Speed: 300 km/h
Range: 620 km
Service Ceiling: 4500 m
Mil Mi-26 Halo
First flight on December 14, 1977, the replacement of the Mi-6 Hook , is the biggest operational Helicopter of the World. Can carry up to 90 troops, a similar capacity of a Lockheed C-130 Hercules and it was the first to operate successfully with an 8 blades main rotor. Variants include:
Rotor diameter: 32 m
Disc Area: 804 m2
Length: 40 m
Height: 8.15 m
Weight: 28200 kg - Max: 56000
Engine: 2 Ioatarev D-136 of
11400 hp each
Speed: Max: 295 km/h
Range: 800 km
Service Ceiling: 4600 m