My understanding is that they are only occupying areas they managed to take.
Rather the original plan seems to have been to install a puppet government in Kiev. And when that failed, they decided to take the easternmost Ukraine + an area around Crimea.
Which makes total sense as their major naval base in the black sea has always been in Sevastopol, Crimea. And they'd want transport to there that doesn't go through a potential Nato country.