Nearly all ethnic Germans who found themselves living on the Polish side of the post-war border moved back to Germany. Hence Germany's population did not decline nearly as much as Poland's as a result of the war, which a Germany started, I might add.
I mean, is it possible to be any more stupid than the Kaczynski twins?
Apparently yes, judging by your comments.
Actually Germany lost even more people as a result of the war than Poland did. Of cause, speaking in generalized terms Germany started the war and Poland was a victim, but the whole what-a-big-country-we-would-be-if-you-did-not-attack-us concept is fallacious and can lead only to a dead end. Past is past, and it's not about to change.