Back then, the USSR was largely a rural backwater. After WWII, there was rapid industrialization, mechanization of agriculture etc. Of course, during the entir

Back then, the USSR was largely a rural backwater. After WWII, there was rapid industrialization, mechanization of agriculture etc. Of course, during the entire cold war the USSR "overspent" on the military compared to the Western economies. But, during the WWII the economy was understandably 100% focused on the war. So even if the kept "overspending" on the military after the war, there was still a lot of industrial capacity left over that could be used for rebuilding and industrialization.