While there are also other reasons for the great progress of USSR under Stalin, it is likely that the most important cause was provided by the benefits that USSR got from being among the victors of WWII.
After the war, USSR incorporated totally or partially many countries and they transformed the remaining Eastern European countries into vassal states.
Especially during the first decade after WWII, USSR has stolen huge amounts of resources from those territories, in various forms, starting with what the Red Army had robbed during their so-called "Liberation" actions, then with huge so-called "War reparations" extracted from the countries that the Soviet Union itself had attacked, so they had been forced to attempt to defend themselves, but eventually they had to pay a lot for daring to do this, and then with various profits extracted from mixed companies established in the vassal states after the war and from various unbalanced contracts for economic exchanges with USSR.
The most affected was East Germany, from where entire factories have been moved to Russia, machine by machine and tool by tool, where they constituted the bases for new industries that were developed in USSR after the war. (While USA had no need to take entire factories to be able to reproduce the German technologies, they also took from West Germany many samples of industrial products, together with their manufacturing documentation, which were given to certain American companies, which then expanded after WWII in domains where previously Germany had exclusivity or domination. An example is the technology of magnetic recording, which became important after the war not only for audio and video recording, but also for the first electronic computers.)
While in some countries dictatorships have achieved economical progress mainly by internal means, under Stalin the greatest achievements were based on plundering most of their former neighbors, while the Soviet Union greatly expanded territorially, mostly at its west, but also at its east.