You can have Stalin restrict german expulsion from the east to the future-GDR areas and have a much earlier Berlin wall, for example built by the 1950. That'd swing the population balance from 51 million to 19 million to 42 to 28 in the 1950, as well as preclude massive emigration from East Germany during the 50's.
This way you'd need a lot less immigration, pro-natal policies or territory acquisition to have GDR being more populous than FRG.