This question is broad, so you’re free to decide which metrics and conditions you consider the most important.
Too lazy to effort post, so these are the numbers off the top of my head:
USSR had about 1/2-3/4 the GDP of the USA in any given year
China currently has between .9 and 1.2 times the GDP of the USA (depending on normalization)
USSR was extremely vulnerable to naval blockade (primary ports were on Baltic & Black seas, trans Siberian rail insufficient for Vladivostok to become primary port) with fleets split into 3-4 major theaters (Baltic sea, black sea, artic sea, northern Pacific)
China is vulnerable to naval blockade (Taiwan, Philippines, Sri Lanka, and Indonesia all sit on/next to China’s major trade lanes), but their fleet only has to worry about 1.5 theater’s (indian ocean, Pacific)
Isn’t GDP just naturally lower due to socialist economy having lower prices, and especially due to free/very low rent, compared to the USA housing market which makes up about 10% of USA GDP?
Yes, also capitalist GDPs tend to get inflated by luxury goods (which the SU notoriously under focused and lacked), but I was/am not prepared to go into a deep dive of gdp/gdppc/gdpppp for base industries for the USSR, USA, and China
Neoliberal GDPs get further inflated by financialization and asset price inflation, things that hinder the real economy. https://michael-hudson.com/2016/08/finance-is-not-the-economy/
Personally the way I’d measure economic strength is simply by multiplying total power generation (industrial development) with total agriculture production (sustainability and general labor output). Data for the USSR is lacking but we can guesstimate that peak USSR was anywhere from equal to somewhat below USA by those metrics while China today is about 1.95 times USA’s size.
An important aspect to consider as well is the lack of an illegal second economy in the PRC due to their socialist market economy. While the USSR may have (emphasis on may, don’t have the stats in front of me) had a larger planned economy at its peak the PRC has managed to prevent any need for the formation of a second economy as was the case (and downfall) in the USSR. If we’re taking strength as in output and stability, the PRC can produce many goods the USSR could not and is preventing a materialist basis for capitalism from forming in a way the USSR never did
