The USSR had a comparable (corrected: not small as I first wrote) population and a small economy compared to the USA's. Moreover their economy was highly disfunctional.
It's a bit like all the fear mongering about Japan in the 80s. The hard truth is that Japan was way too small to be a threat.
China, on the other hand, is much bigger than the USA. The fact that the USA are currently so much stronger than China is therefore the anomaly in a historical perspective.
The USSR was the 3rd most populated country in 1990, ahead of the US. The US census calculated 248 million in 1990 vs 286 million for the USSR in 1989.
It's a bit like all the fear mongering about Japan in the 80s. The hard truth is that Japan was way too small to be a threat.
China, on the other hand, is much bigger than the USA. The fact that the USA are currently so much stronger than China is therefore the anomaly in a historical perspective.