That's a big drop in population since nearly all countries gain population. This will probably become a common sight, especially in Europe and Japan, where there is a aging population
It's not really a real drop though.
81.7m was only an estimate while the 80.2m figure is more factual.
The last census was made in 1987 and that was before Germany was unified and it didn't drop from there.
Also according to other federal sources, population has gone slightly up in the past few years due to more immigration.