Hiya!

The vast majority of German populations east of the Oder-Neisse, as well as in the Czech Sudetenland, were not 'lost' in terms of 'becoming foreign citizens.' Rather, they were forcibly relocated en-masse into what would then become the 'new' German border between January, 1945 (largely as refugees fleeing the approaching Soviets) and in a more organised manner in the aftermath of the war. The first wave of Eastern refugees before the war's end suffered horrific privations. Many died of hypothermia and starvation during the early months of 1945, and many more were murdered by advancing Soviet soldiers in retaliation for the atrocities visited upon Eastern Europe by Nazi Germany and its allies. With the end of the war, the occupying powers decided that remaining Germans who had not fled from regions now part of modern day Poland and Russia (namely East Prussia and the more densely populated regions around Danzig), as well as large ethnic German populations in Czechoslovakia were to be forcibly relocated into the new occupied zones. Relocated Germans had to abandon their homes and most of their remaining belongings, and had no choice in where they were resettled. The massive upheaval that resulted continues to be part of the German post-war identity.

Consensus puts the number of forcibly relocated Germans by mid 1946 at ~14 million, or roughly 25% of the total postwar population of Germany. However, because of the extremely chaotic nature of a lot of these relocations - particularly those during early-mid 1945, it is difficult to determine exact numbers of how many were relocated, and how many died. I've seen figures of between 700,000 and 2,000,000 deaths among displaced Germans between 1945-1946, the vast majority of whom died or were killed during the Soviet advance between January and May, 1945.

The mass ethnic cleansing of Germans/German-speakers from East-Prussia and previously-German territories, combined with the generally horrific conditions on the European continent at the end of the war, created arguably the largest refugee crisis Europe has ever seen. However, the statistics you've come across that put Germany's population at roughly the same levels (though I'd be cautious of any statistics that claim to be overly firm!) in 1945-1946 are no great surprise. Those Germans living in territory that would be annexed with the defeat of Nazi Germany remain Germans - they were expelled, not naturalised into their new states.

I'm sorry I can't comment on the effects of the loss of territory on World War One, but I hope this helps!

Source:

Piskorski, Jan M. Die Verjagten: Flucht und Vertreibung im Europa des 20. Jahrhunderts. (The Hunted: Flight and expulsion in 20th century Europe.) Munich: Siedler Verlag, 2010.

I can't speak for the now - French region of Alsace, but a major (and pretty ugly) removal of German speakers from eastern Europe pushed many of them out of areas that had been German-speaking for hundreds of years to fit the new "national" boundaries; much of it was misplaced anger at the Germans living otherwise peacefully in the urban areas ands far as Hungary, and these people were harshly mistreated as their possessions were stripped and they were sent to a new and already struggling nation.

https://en.m.wikipedia.org/wiki/Expulsion_of_Germans_from_Czechoslovakia

This wikipedia article cites 1.6 million Germans being expelled from Czechoslovakia alone, and I believe the same happened in Poland and other places. A more chilling quote was the Czech government's phrasing of the policy: "a final solution to the German question", chillingly reminiscent of Nazi euphemisms for their own 'removal' campaigns.