The Russian Church alone counts as half of the members of the entire Orthodox Church. To emphasize the very function as world leader in the Orthodox world after the fall of Constantinople (1453) the patriarch of the Russian Church named itself patriarch of the “Third Rome”.
Its canonic territory covers, according to the Moscovian point of view, all of the ex-U.S.S.R. That is not shared by other Orthodox Churches and is the origin of disputes and misunderstandings particularly with the Ecumenical Patriarchy (see in particular the Estonian Church and the Ukrainian Church) and with that of Romania (relative to the Orthodox jurisdictions in Moldavia and Bokovina) and at times even of schisms (in Ukraine).
That same Russian Church further is divided since in 1921 the Russian Orthodox Church was formed outside of Russia (R.O.C.O.R.). After the fall of the Soviet Union, the debate for the re-unification became lively, but till now useless the attempts to re-unite this Church in Moscow because she considers the political ecumenicism heretical, and considers the Patriarchy still an effect of Sergianism.