Look at the growth rate in the US during the baby boom. The population grew 18.5% between 1950 and 1960, and that's without mass immigration. Now take away equal rights for women in the decades after that, and ban abortion, and the pill when that becomes available. Then indoctrinate your citizens the importance of getting married and having big families from a young age, and offer economic opportunity and large plots of land if they move east.
I don't see why they couldn't get something like 18% growth in the 50's, 15% in the 60's, 12% in 70's, 10% in 80's 8% in 90's, 6% in 2000's, 4% in 2010's. With a starting population of 80 million Germans and another 30 million Germanic peoples from Scandinavia and the Low Countries, plus another 20 million Germanized Slavs, that's 130 million starting number. By 2017 they'd have around 260 million in the Greater Germanic Reich.