The view of the Soviet Union as completely annexationist at all times is a false one, coloured by Cold War narratives. Soviet foreign policy under Stalin was in many ways Russian imperial expansion revisited. Consider many of the territories annexed to the Soviet Union during the war: portions of Finland (lost 1918), Bessarabia (lost 1918), the Baltic Republics (lost 1918), Eastern Poland (lost 1921). Carpathian Ruthenia (i.e. the eastern chunk of Czechoslovakia) was an exception, but it had the double feature of a) having a majority population of Ruthenians, closely related to Ukrainians, and b) giving the USSR direct land access to Hungary, a country they had placed in their sphere of influence. Consider the issue of Finland in particular. Before the Winter War, the Soviets were willing to give up territory (albeit largely worthless territory) in exchange for the Finnish land they desired: they did not demand that Finland completely surrender. If the Soviets were only interested in grabbing every bit of land they could acquire, or even just every bit of land they could conceivably lay claim to (which would have included Finland as a whole), this would not be the expected negotiating position.

In terms of Poland specifically, a full annexation of Poland was actually the desire of many of the communist Poles. The older generation of communists often had older notions of communist theory and practice, ideas that fell by the wayside as communism increasingly came to mean "socialism in one country" and thus "the USSR". For these communists, nationalism was an obsolete and poisonous doctrine, which made the idea of separate states in a communist world redundant and even potentially harmful. The communist Poles thus lobbied Stalin for Poland to become a republic within the structure of the USSR, rather than existing as a separate state, even one that would have been communist. Wanda Wasilewska and Alfred Lampe, probably the two highest-ranking communist Poles in the USSR, were of this opinion. But even with Polish communist support, Stalin ultimately disagreed. He committed himself to the Western Allies as permitting the existence of an independent Poland as early as Tehran in 1943, albeit one who territory was radically shifted. He would be content with a communist puppet government, the annexation of the Polish Eastern borderlands to the USSR (with their majority Ukrainian and Byelorussian populations), the subsequent ethnic cleansing of Poles and Ukrainians from each others' revised territories, and the shifting of the Polish border westwards with the Polish annexation of those German lands the Polish communists referred to as the "Recovered Territories". That the USSR didn't attempt to absorb any of the eastern European nations that were not historically part of the Russian Empire, even those that the Western Powers cared much less for, shows that the example of Poland fell within established patterns of Soviet postwar foreign policy: puppets, ethnic consolidation, and enforcing (some) historical claims.

EDIT: accidentally hit the submit button mid-post and now have to edit this to completion. My apologies.