Towards the end of WWI, both Austria-Hungary and Russia faced populist or socialist revolutions and successful nationalist uprisings. In both cases, both revolutions and uprisings resulted in the creation of numerous successor states. In both cases, war existed within and between successor states.
In the case of the successors of Imperial Russia, the new states generally united.
In Union as of 1922: RSFSR; SSRS: Ukraine, Byleorussia, Armenia, Georgia, Azerbaijan,
Non union: Finland, Lithuania, Estonia, Latvia, Poland
Major wars: Russian civil war first (internal) and second (intervention) periods, with warring successor states above, and failed alternatives to the above
In the case of the successors of Austria-Hungary, the new states did not generally unite.
In unions: Czechoslovakia; Yugoslavia
Non unionate: Romania, Hungary, Austria, Poland, Ukraine, Italy
Major wars: Hungarian revolution intervention, Trieste, Russian Civil War
In particular, the central economic mass surrounding the dominant nations of Imperial Russia formed a union, whereas in Imperial and Royal Austria Hungary the peripheral nations formed the only unions.
Why the difference?