Based on the near universal view that 'big' states can develop in lop-sided federations; they pay for everything, they put in the effort, other SSRs are scroungers etc.
Remove populist nationalists like Yeltsin from the picture in both Russia and Ukraine and you've got a shot of the Union State surviving. I believe Yeltsin and several contemporaries from other major SSRs ignored the referendum and unilaterally declared their independence from the USSR to continue their personal goals. No Russia, no Ukraine, no Belarus and a *USSR state is dead in the water. Though I'm surprised more wasn't done to create a Central Asian Federation after it all fell through.