I've written a bit about the integrity of Soviet economic data before. Yes, the headline growth figures were inflated, albeit typically by statistical sleight of hand and rarely outright falsified. Which makes pulling together these sorts of international comparisons tricky at best.

That said, there's nothing particularly startling about the Soviet figure for 1965. Whatever about the exact numbers, which I'd have little faith in, the Soviet economy was in rude health until the mid-1960s. The post-war recovery had been surprisingly swift, largely achieved by 1950, and what followed was what RW Davies has called "the golden years of the Soviet administrative economy" (Soviet Economic Development from Lenin to Khrushchev). It saw a sustained period of growth (GNP doubled in 15 years) and a reorientation towards consumer goods that significantly improved living standards for the population. This was the optimism that made Khrushchev's boasts of burying the West seem credible and bound the emerging 'middle class' to the state, ie Vera Dunham's 'Big Deal'.

What followed from the crest of 1965 was decades of stagnation and eventual collapse. The drain of defence and agriculture, stubbornly low productivity, increasingly complexity of a maturing economy, failure of reforms, etc, all slowed Soviet growth to a crawl and then a crisis. I can't talk with any confidence on the post-Soviet economies.

But at the same time, and this is what your comparison is probably missing, the rest of the world economy was continuing to grow. If we assume that the rest of the world followed the US example and grew seven-fold between 1960 and 2010 (it didn't but go with it) then even maintaining low growth rates to the present day, with no collapse, would have seen the Soviet percentage share shrink dramatically.

So before even considering the breakup of the USSR, we need to accept that from the mid-1960s the USSR was simply being outpaced by the growth of others. As the pie got bigger, the Soviet contribution to this got smaller. Ditto for China. A percentage share of the world economy depends as much on what happens elsewhere as in the nation in question.

Note that you are quoting the share of the world economy and neither China nor Japan had developed into economic superpowers by 1969.

Hence the "shrinkage" is not necessarily because the Soviet economy's size was overstated, not is it even necessarily true that it shrunk. Instead what this means is that the Soviet economy simply failed to grow at the same pace as China/Japan.

China in 1820 likewise did not yet have to compete with a fully industrialized Europe - whose economies grew by an enormous pace during the Industrial Revolution whereas China's economy stagnated or even shrunk (multiple massive civil wars and foreign invasions will do that to an economy).

To get a real view of whether the economies grew or shrunk you need a study of their GDPs adjusted for inflation.

As an aside to my other comments I would very strongly urge everyone to be very wary of Maddison's data or any similarly specific computations (including Bairoch's, which were more creative and possibly better-founded if we knew just how they were constructed).

Maddison's early assemblages of modern growth rates were valuable, their later mutation into purported real GDP at 1990 prices going back to AD 1 decidedly less so. There really are no 32.9%'s in 1820, for anything.

Just to illustrate, the Chinese internationally-comparable GDP on which Maddison's purchasing-power parity-based projections were anchored was nearly halved by the World Bank in 2005, only to be revised part of the way back in 2011. It really is that crude.

So don't read too much into any claimed aggregate or percentage before 20th-century national accounting, least of all one in "1990 Geary-Khamis dollars": economic activity in 1820 wasn't denominated in 1990 Geary-Khamis dollars.

Soveit Russia didn't get back to it's pre-war industrial output after WWII until the 1970s and never came close to matching the U.S. in output. Their numbers by the 1980s where so over inflated as to be not only useless but comical. By the end many in senior leadership positions were little more than vodka soaked zombies who were barley functional.