I'd say that the Soviet Union was more powerful at its peak in the middle of the 20th century than Russia ever was in the 19th century or any other period. After WW2, all of Russia's 19th century rivals (save Britain, in part) were badly weakened (Germany, Japan) or completely out of the picture (Austria-Hungary, Ottoman Empire), with the Soviet Union's influence extending farther than it ever had. Certainly, the post-WW2 Soviet army was more powerful vis-a-vis its land rivals than the Russian army ever was. I'm no economic historian, but I'd also guess that the Russian/Soviet Union share of world GDP peaked at some point during this period, reflecting the extent to which the Soviets had been successful at modernizing (albeit still imperfectly). The development of nuclear weapons also gave the Soviet Union a gigantic military advantage over all but a handful of countries.
I do think there's a counter-argument to be made that, while the Soviet Union was clearly in the top 2 in the middle of the 20th century in a way it wasn't in the 19th century, the distance between it and the #1 power (the U.S.) was greater at that point than it was in the 19th century (at least post-Napoleon). Maybe another argument is that the Russian Empire held up reasonably well for 300 years (from the Time of Troubles to the Russian Revolution) whereas the Soviet Union lasted around 70 years; one might argue that this points to an inherent weakness in the Soviet system.
The biggest weakness for the Soviet Union was that it was built on singular adherence to a flawed ideology and an ideology that really loses its relevance in the post industrial world. As the rest of the globe shifted to a more service based economy, the USSR could not because it had built up its heavy industry, and neglected its consumer economy. It is hard to have a service dominated economy, with a small high tech manufacturing sector, when you have no consumer culture, no personal agency and depend on an dogmatic ideology that sees the "end of history" as being the industrial state.
This is how the USSR fell behind Japan and I believe West Germany as well during the 70s and 80s, before the breakup even happened. The breakup then hammered the final GDP of Russia since so much territory was lost and then you have all of the power breakup economic upheaval, the rise of the oligarchs, the offshoring of wealth and dependence on the oil and gas sector.
The USSR also took its unifying identity from adherence to said ideology, and this is the point that Putin likes to make (and abuse obviously) that the USSR decentralized the nominal government structure in the name of appeasing sectionalist nationalist, then relied on the centralized authority authority of the Party to maintain Tsar like power in the government. Thus the unity of the enterprise hinges on the continued presence of the Communist Party and its continued power, which is staking a lot on the solidity of such a flawed enterprise and surely the end result seems rather obvious once such ideology burns itself out in terms of credibility.
Of course in contrast to Putin, I do not believe that the greatest geopolitical disaster of the 20th century was the breakup of the Soviet Union, as any Conservative would and arguably by definition should see the world, that designation belongs squarely to the Russian Revolution.
The Soviet's were the architects of their own demise in so many ways.
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