There's two very divergent possibilities here. It could be isolationist with something like China's own self sustaining culture bubble, or it could be heavily invested in Europe (or more unlikely, Asia) and thus share a lot of what it produces, or even outright dominate culture markets like the US does.
Also looking at your curve, what you need to avoid is WW1 and WW2, not just communism and nazism. The first decline starts with the WW1 slaughter, not the revolution. So I think that's your easiest route here. Find a different spark for WW1 that keeps Russia out of it.
Actually, when Russia's population growth started slowing in its tracks dates back a bit further than that, to the late 1800s, and the repercussions of the Prussian deportations of 1885-1890, personally championed and initiated by Bismarck, as well as the Prussian Settlement Commission and the anti-Polish post-unification Kulturkampf policy which he enacted, owing to his own personal belief in the existence of a widespread Catholic conspiracy, plotted by what he called the "Coalition of Catholic Revenge" (France, Austria, and the Catholic Church itself), which posed a threat to both his German and European policies. As a result of Bismarck's efforts to ethnically cleanse Germany of its Polish (and Jewish) minorities, combined with growing Nationalist and Pan-Slavic trends in Russian politics, the formerly good relations between Prussia/Germany and Russia greatly worsened in the 1880s, with the backlash increasing negative sentiments against German minorities in the Russian Empire.
With that in mind, the German ambassador in Russia, Schweinitz, advised Bismarck to abstain from further expulsions, anticipating that they would only provoke the supporters of Pan-Slavism and trigger repressions against all German settlers in Russia. And soon afterward, after Bismarck continued his expulsion policies, the Russian government imposed legal restrictions on acquisition and lease of land by Germans in Russia, thus limiting the German colonization movement in the Russian-controlled part of Poland, which had previously been their primary destination of choice, and fuelling a massive boom in immigration to the USA from Eastern and Central-Eastern Europe. Contrary to Bismarck's original intentions, the expulsion contributed to the worsening of German-Russian relations and the erosion of their long term cooperation – resulting in a shift in Russia's external policy which finally led to the creation of the Franco-Russian Alliance, soon transformed into the Triple Entente, which fought the German Empire during World War I in 1914–1918 IOTL.
So, looking at the target here, of a 'Greater Russia' with over 500M people, it's better to go back a little further than WW1. From 1850 to 1910, even IOTL, the Russian Empire's growth had been the fastest of all the major powers except for the United States, having doubled between 1850 and the Russian Imperial Census of 1897, from c.60M to 125.6M people. And this represented a marked slowing of growth from its peak growth rate, with the population growth rate of Imperial Russia having rivaled that of the USA between 1850 and 1880, largely courtesy of the far-reaching government reforms of Tsar Alexander II; however, after his assassination by populist, anti-modernisation Slavophile revolutionaries in 1881 saw his successors initiate a period of political counter-reforms from this point onward, which saw a great increase in police brutality and the major suppression of civil liberties in Russia.
IMHO, looking at the timeline, and the easiest route to achieve this AHC, I'd go for butterflying away the assassination of Tsar Alexander II, and the continuation of his reforms in Imperial Russia (which his son Alexander III reversed largely out of fear for his own life, having had his coronation delayed by two years due to the ever-present threat of his own assassination, and the belief that only by remaining true to Russian Orthodoxy, Autocracy, and Nationalism, could Russia be saved from revolutionary agitation, and he avoid the same fate that befell his father). Perhaps as a result of Bismarck's pogroms having been delayed or dialed back somewhat, moderately weakening the populist support for Nationalist and Pan-Slavic agitation in Russian politics ITTL than IOTL?
As such, if Imperial Russia continued its trend of population growth from 1850-1880, without the mass emigrations which it saw predominantly to the USA IOTL (with the Prussian-Russian Alliance holding steady ITTL, with both German minorities in Russia and Slavic minorities in Germany predominantly being incentivized to return to Germany/Russia instead, and without the anti-Jewish riots and pogroms in the aftermath of Alexander II's assassination which forced a sizeable share of Russia's Jewish community, which had been and still remained the largest in the world, to flee from the Pale of Settlement, a vast area in the Russian Empire where Jews were allowed to live and where they dominated market-intermediary occupations, such as trading and moneylending), it wouldn't be wholly unrealistic for the Russian Empire to have a population of 200M+ by 1913 (up from 170M+ IOTL).
And even IOTL, Imperial Russia's share of the world population had increased from c.3.6% in 1800, to c.5.6% by 1860, c.7-8% by 1880, and c.9.8% by 1913, before commencing its population decline relative to that of the global population; ITTL, it'd be realistic for more than 10%, and possible for more than 12% of the world's population to live under the rule of Imperial Russia by 1913 (including the overwhelming majority of the world's Jewish population, who had comprised as much as 16-18% of the general population in the Pale of Settlement under the rule of Alexander II, which covered a vast area in Eastern Europe, including parts of contemporary Latvia, Lithuania, Poland, Russia, and Ukraine, and the whole of contemporary Belarus and Moldova). And IMHO, it would've had a decent chance of retaining a share above 10% to the present day, especially if WW1 still happens ITTL- with the continued existence of the Prussian-Russian Alliance being most likely to result in Prussia and France subsequently trading places in TTL's equivalents to the Triple Entente and Central Powers.
And I know which side I'd be putting my money on to win ITTL; with TTL's Russia, allied with Germany and the British, being ideally poised to greatly expand its own borders, and population, with little to no concerns of invasion into Russia to worry about thanks to its alliance with Germany effectively providing it with the strongest possible buffer state against its most powerful potential adversaries, and leaving it with only the Galician, Caucasus and Odessan Fronts to worry about. Especially if the Ottoman and Austro-Hungarian Empires still fall, while the Russian Empire endures without collapsing- how much of their territories, and their populations, could end up potentially falling under Russia's control?