Im not well enough informed about where the border should be between EUkraine and Little Russia. I imagine the population of Kiev has changed over the years since the creation of an independent state, though I agree historical Kiev would have been Russian.
The point is, the current Ukrainian State is too large and has the wrong borders.
I believe if we are to look from a very pro-Ukrainian but still somehow objective perspective the proto-Ukraine would be Zaporozhskaya Secha - a semi-authonomous Cossak statelet existing on a very small territory around lower Dniepr River. It did exist in some form between late 15th and late 17th century as a buffer zone between Poland, Ottomans and Russia but mostly Russian-controlled. It disappeared around 1770s.