Quibble: At first, Berlin was split into four occupation zones: Russian, American, British and French.

While that doesn't answer your question, It's interesting to note nonetheless.

At the Potsdam Conference (and previously discussed at Yalta) between the "Big Three" powers (UK, USA, USSR) agreed to split Berlin into zones of control between France, UK, USA and USSR in an effort to address some sticky post-war questions. The Soviets were still deeply suspicious of the western powers and wanted a big buffer zone between themselves and any potential rivals. The allies all agreed to pursue a policy of German disarmament and dismemberment so that Germany could never stand alone again and threaten world peace. The western zones eventually united to form a combined West Berlin. The Soviet section was roughly as big as the other three zones combined.

Months before Germany surrendered, Allied leaders were already planning how the allies should resolve the end of the war. At the Yalta Conference, they agreed to split up Germany into four zones, and Berlin as well into four zones. It was a compromise to ensure that no occupying force got too much. If you are interested in more, read up on the Yalta Conference and the European Advisory Commission.