Thanks Maksim!

The data is from Our World In Data [link]. You are right that the GDP/cap starting point is lower in USSR ($3K) vs US ($15K), but the size is not the reason for relative acceleration as you can see from the volumes here:

As to why USSR saw (a short run of) success, this article explains it best. Excerpt here: “For 40 years the Soviet Union was indeed a growth miracle, but it was a spectacularly unsustainable one based on extractive political and economic institutions. The powerful Soviet state could generate large productivity increases by moving people from rural areas and putting them into factories. But the system totally failed to generate incentives for improving productivity or for innovation except in military areas where they put a huge amount of resources.”