Finland decreased its poverty between 1917 and 1991 more than Soviet Union did. In the beginning of year 1917 Finland was a part of the Russian Empire, so we were extremely poor here as well. Soviet Union could be on the second place, perhaps. But, since there is at least one country that fared better, the claim you made it evidently false. There can very well be other countries than just Finland that decreased poverty more than USSR did. I do not know for sure, though, as I'm not terribly well aware of how faraway places like Chile or Burma were faring in 1917.