No doubt about it--the 90 sq. ft. cabins are small. Yet I have some fairly large clients (some over 6 feet tall and/or well over 220 lbs.) who have stayed in them and had no complaints.
Most people do not spend a lot of time in their stateroom, and therefore they reason that they'd rather spend "discretionary funds" on a lovely keepsake (like a lacquered box or a replica of Faberge egg) instead of on a roomier cabin that might also be available to them (Viking has 200+ sq. ft. staterooms on the same ships).
And, although some of the cabins are quite small, Viking's ships have other amenities which you can enjoy: a roomy, comfortable library; a massage by a masseuse on board; two large lounges, which offer live ongoing entertainment every evening; a fairly large souvenir shop; two dining rooms; large outdoor decks, etc.
And, another rationalization for opting for savings on the smaller cabins: you really go to Russia for the destination--to see and experience Moscow and St. Petersburg and all the quaint villages in between. The shore excursions are the important elements of a cruise in Russia. With that in mind, Viking's onboard guides and the local guides in each port are extremely knowledgeable and personable--they make the cruise experience in Russia what it is: awesome!