It probably was much larger than average Soviet house at the time, as well as far more luxurious. Stalin was into expensive cars and fine cuisine as well (even as parts of the country quite literally starved to death).

Thing is, you don't have to be ultra-rich to feel good about yourself. You just need to be substantially richer than the next guy. And no party functionary (let alone Stalin) ever had any lifestyle issues. Dachas, yachts, personal chefs - it's par for the course until your fellow party members take you out with a pickaxe or something to free up the spot.