it is not question of nationality it is question of law and previous border agreements.

Many Ukrainians live in Kuban, but it is Russian territory. i know that many Ukrainians in Kuban were forced to change their surnames to russian. so it is not realistic to sort out which nationality is the majority on which territory and to whom it belongs. 
Just in 1944 a large number of Russians were brought to Crimea. that is why maybe (can't say for sure) their are majority. after annexation also thousands of russians were moved to crimea.
I lived in Abkhazia, it is ancient Abkhazian territory, but at that time Abkhazians were about 20 percent there, so should we give the territory to the national majority too?