Between 1991 - the first full year after reunification - and 2013, the population of east Germany fell by two million, the federal statistics office Destatis said in a report entitled "25 Years of German Unity" ahead of the anniversary on Saturday.
The weaker economy sent many east Germans fleeing west in search of jobs, and drove the birth rate down far below replacement.
While 3.3 million easterners moved west, only 2.1 million west Germans moved to the former communist east during that period, the study found.
By 2013, Germany counted 12.5 million easterners, down sharply from 14.5 million in 1991.
"The fall of the Berlin Wall (in 1989) sparked a strong emigration movement," Destatis president Roderich Egeler told reporters.
However the trend slowed significantly in recent years, and in 2013 for the first time more people moved from west Germany to Berlin and the east German states than in the other direction.
Meanwhile the east has suffered from a dramatic greying of its population and higher unemployment.
In 2013, nearly two out of three people were older than 40, up from less than half in 1991.
With the collapse of the state-run economy, the labour market also saw dramatic upheaval, dragging employment rates and disposable income down with it.
Per-capita gross national product in the east is still one-third that in the richer west while joblessness at just under 12 percent is nearly twice as high as in the west. And while the west is largely powered by industry, the east relies almost entirely on the lower-paying service sector.
The Berlin Wall fell in a peaceful revolution on November 9, 1989, ending four decades of post-war division and paving the way to reunification 11 months later.