Check that: http://en.wikipedia.org/wiki/History_of_Ukraine
Basically South and East of Ukraine were not inhabited by Ukrainians, but colonized by Russian Empire - Odessa, Nikolayev, Crimea.
And by the way, Holodomor advocates usually forget to mention that many more Russian and Kazakh people died during that famine and it was not specifically 'targeting' Ukrainians - http://en.wikipedia.org/wiki/Soviet_famine_of_1932%E2%80%931...