Just to clarify, they did not annex Ukraine or Belarus after WWII. Those two were already constituent republics of the Soviet Union, they simply gained new territory taken from the pre-war Second Polish Republic, mostly based on large concentrations of ethnic Ukrainians and Belorussians in the annexed regions (as well as Ruthenia, snagged from Czechoslovakia). As for the Baltic States, they had belonged to the Russian Empire for years prior to its collapse during the Russian Revolution and ensuing Civil War, and so there was an irredentist claim to be made there.

Romania at least was under soviet occupation until around 1951, and had part of its territories directly annexed by USSR (present-day Republic of Moldova) but unlike Poland did not get new territories in exchange