From an earlier answer of mine

The answer to this question really lies in the differences between the Russian Revolution and Civil War and the Second World War. Both conflicts not only took place in different geopolitical contexts, but also in a different ideological environment that conditioned Soviet responses to the national question and territorial formation.

The national question caught Lenin and company somewhat flat-footed in 1917. As Marxists inside an active revolution, they were naturally disinclined to give much weight to the national issue. In classical Marxism, nationalism was a tool of the bourgeoisie and monopoly capital to distract and divide the proletariat from achieving true class consciousness. Although the turn of the century saw the emergence of significant models of how nationalism would fit in a worker's revolution (eg Austromarxism, Cultural-National Autonomy, Personal National Autonomy, etc.), the Bolsheviks found after their seizure of power that they now had to deal with the nationalities question with concrete policies. The Russian empire, whose various state organs were disintegrating as the Bolsheviks stormed the Winter Palace, was majority non-Russian, but a good portion of the Bolshevik leadership and rank and file were Russian or Russified. The result was that Bolshevik leadership tended to move on the nationalities question with a mixture of tactical declarations with the aim of using nationalism to secure the revolution.

This was not entirely successful at first. Lenin boldly declared after the November Revolution that the Bolsheviks championed the right of national self-determination with the aim of encouraging wider revolution. When Finland declared its independence in December 1917, the Bolshevik leadership accepted it in the hopes of that a Finnish communist party would take power, but an anti-communist government took power in Helsinki after a brutal civil war. Likewise experience in the Baltics, where anti-communist nationalists tended to take power with German assistance, meant that Lenin needed to condition his call for national self-determination. This meant Bolshevik responses towards the national issue were often tactical compromises with a clear agenda that nationalism could not work against the wider interests of the revolution.

The historian of tsarist nationalities policies Eric Lohr has described the eighteenth-century tsarist approach to immigration as "attract and hold" meaning that they induced foreign colonists like the Volga Germans to settle and then kept them inside the empire through a variety of state levers. "Attract and hold" is also an apt metaphor for to post-Finnish Bolshevik nationalities policies. The Bolsheviks encouraged the revolution in the vast non-Russian empire and appeals towards nationalism was one of the tools in the Bolshevik's arsenal. The Bolsheviks simply could not ignore nationalism in light of the Civil War lest it be instrumentalized by the Whites. Leaving national movements alone risked another Finland. In Central Asia and the Caucuses, Lenin crafted an appeal to nationalist sentiment by dividing nationalism between exploiter and exploited nations. Bolshevik agitprop materials often emphasized the Great Russian chauvinism of the empire had encouraged a healthy counter-response among the exploited non-Russian masses. However, the Bolshevik approach always called for some sort of mediation of nationalism by the emerging Communist Party. This bifurcated approach towards nationalism, attracting non-Russians with promises of national justice but holding them within an ideological structure, became one of the hallmarks of Soviet nationalities policies. The 1920s saw the introduction of korenizatsiia (indiginzation or as Terry Martin terms it "Soviet affirmative action) in which the Communist Party sought to raise up native cadres of Communist activists inside their own territory. Although the state walked away from korenizatsiia in the 1930s, it never really went away in the USSR and would remain part of the Soviet policy until the end of the USSR in 1991. This imparted a certain political stability to areas like Central Asia in which local elites assumed positions of power within the Communist Party apparatus.

By 1944/45, it was a much more different situation. Although to be clear, the Soviets did annex significant territory from Finland, Poland, Romania, East Prussia, Czechoslovakia, and the Baltics, so sizable portions of Eastern Europe did end up inside the USSR. But Stalin's basic agenda for Eastern Europe was to expand the Soviet sphere of influence and secure the Soviet state (and these two precepts were pretty much indistinguishable in practice). For areas under direct Soviet influence, this meant encouraging local Communist parties to take power, through legal and extra-legal means. Moscow could count on a wide number of emigre Communists to take these posts and nationalism was one of the tools used by these satellite Communist parties to cement their rule. While Communist-style nationalism had yet to achieve the prominence as a legitimization stratagem it would in the 1950s, wartime and postwar propaganda produced by these Eastern European Communist parties stressed a popular front and a desire for the nation to rise up and cast out the foreign German invader and their collaborators.

Preserving national borders, albeit altered for Soviet purposes, also fit into the larger geostrategic designs of Stalin. Not only would massive annexations further rupture the wartime alliance with the Western Allies, it would also be counterproductive for the wider postwar Communist movement. Stalin expected, with some justification, that Western European Communist movements would achieve broad electoral gains in the immediate postwar period. Broad annexations in Eastern Europe would hurt this chance as nationalism still exerted a powerful force and French and Italian Communists were still French and Italian. The time was not yet ripe for the erasure of the nation and its replacement with a Soviet identity.

Germany was also a special case in Soviet calculations. Although the hindsight imparted by the Cold War has made German division appear a foregone conclusion, things were not so clear in 1946/46. The occupation and military governments were explicitly temporary and Allied wartime agreements held that Germany would be reunited once all the Allies signed a proper peace treaty with an acceptable successor German government. The KPD, later renamed the SED, as well as the Soviet military government placed emphasis in their electoral propaganda that a KPD/SED list was a step against a Western-led division of the country. In reality, Moscow as well as Washington shared blame for German division, as did the various postwar German parties.The Soviet zone's media did emphasize the justness of the Oder-Neisse line and the Soviet annexations of East Prussia, but often in ways that framed these territorial revisions as essential for the postwar order and peace.

Things, of course, did not play out in the postwar period as Moscow expected. Nationalism became one of the Achilles' heels in Eastern Europe and there was a widespread sentiment that the ruling elite did not really have the nation or the people's best interests at heart. The Soviet's use of hard and soft power to rig Eastern European elections for Communist victory likewise denuded the Communism of much of its postwar prestige in Western Europe. Ironically, the slightly higher standard of living within Eastern Europe and compared to the USSR created a degree of resentment within the wider USSR in the 1970s that their satellites were living better than the leading partner in the Communist experiment that had to bear the burden of being the elder brother in the family of socialist nations.

Sources

Abrams, Bradley F. The Struggle for the Soul of the Nation: Czech Culture and the Rise of Communism. Lanham, Md: Rowman & Littlefield, 2004.

Sygkelos, Yannis. Nationalism from the Left: The Bulgarian Communist Party During the Second World War and the Early Post-War Years. Leiden: Brill, 2011.

Smith, Jeremy. Red Nations: The Nationalities Experience in and After the USSR. Cambridge: Cambridge University Press, 2013.

Suny, Ronald Grigor. The Revenge of the Past: Nationalism, Revolution, and the Collapse of the Soviet Union. Stanford, Calif: Stanford University Press, 1993.

Part of the USSR's goals was to annex those parts of old Imperial Russia that had been lost in the Treaty of Brest-Litovsk. Also, Stalin wanted to create a buffer zone between Moscow and the rest of Europe, considering how close Moscow had come to falling in WWII.

Also, regarding the USSR's goals of spreading the world revolution...yes, Marxism promotes internationalism, but on the whole the USSR ended up pursuing a "socialism in one country" policy, which meant that it often was not especially active in promoting the world revolution.

What you should consider is the definition of eastern Europe. Either it's just the eastern slavic nations and the baltics, or those plus what some consider central european; former Czechoslovakia, Poland and Hungary. Of these, only Poland was part of the empire.

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