USSR didnot traded much outside its block. so very little is known how they calculate things like services.I don't think that on a PPP basis, the GDP of the USSR ever exceeded 50% of America's, making China the only real peer-competitor of the US in history.
what USSR achieved it built its own airline industry and was energy independent across the block. Aeroflot became world largest airline even though alot people not capable traveling outside. what USSR provided to countries like West Germany, Spain and Netherlands to be alot more independent from US.
US GDP is very different now. you have to count Mexico and India part of US GDP. India software services are overhelming designed for US. similar Mexico industries. these things can create alot of GDP inside India and Mexico. US GDP would be alot larger if all those things and people associated with it are imported into US.