"Akzeleration" is exactly that: acceleration, just like in a car. The paragraph is talking about the development of the human race as applied to Germany. Put simply, it will mean that the Germans are still getting taller, but this vertical growth is slowing down while their body mass is on the rise. In other words, they are getting taller slowly, but fetter quickly. :-)))) NO OFFENCE MEANT! Ich bin auch klein und nicht sehr schlank!