Donbas is historically part of Russia, they didn't move in Russians they were already there. You don't get an area that size to 92% by moving them in. Ukraine was literally created as a state by the USSR. Khrushchev made the decision to attach Donbas and Crimea to it.
Ukraine wasn't even a country until the early 1900s at all. It was historically part of Romina and part of Poland and parts were at times unclaimed territory. The word Ukraine itself means "borderland".
The entire mess today is more of what happens when people draw lines on a map to suit their own goals. Why did Khrushchev make Crimea part of Ukraine for instance? Well since it was all the USSR anyway and since Khrushchev was from Ukraine and wanted to retire in Ukraine he attached Crimea to Ukraine.
If you look at the world anywhere we have a lot of warfare some fuckers some where played with borders outside of where they naturally fell along cultures and common beliefs.
The entire mid east, the Korean and Vietnam peninsulas, Eastern Europe, etc. It is almost like changing borders to suit the agenda of people outside of them is a bad idea or something. Kind of seems like three assholes (Rosevelt, Churchill and Stalin) divvying up half the world 70 years ago is still fucking the world over today.