The answer is not that interesting, I'm afraid. The Ukraine was part of the Soviet Union, the Soviet Union won the war, and so Stalin was largely free to settle outstanding Eastern European border issues to his heart's content. Areas seized were either those the Russian Empire had some claim to, or areas that contained a significant number of Soviet constituent peoples; revisions benefitting the Ukraine ultimately benefitted Stalin and the Soviet Union.

As such, Poland lost a bunch of land because the areas seized had large numbers of Ukrainians (and Byelorussians), Czechoslovakia lost Subcarpathian Ruthenia (full of Ruthenians, gave USSR direct land access to Hungary), and Romania lost Bessarabia and much of Bukovina (former Imperial holdings; bordered Ukraine).

It was only logical to assign most of these territories to the Ukraine, as they contained Ukrainians (or did after the post-WWII resettlements). Stalin and other Soviet leaders did not plan on their state breaking up (just like any other stable state), so the assigning of lands was performed from the basis of what was most logical from an administrative basis; it would have been silly to have assigned them to the Russian SFSR when they were not inhabited by Russians and not contiguous with Russian SFSR lands.