And lost 70,000 square miles to Soviet Union for a net post war loss of 30,000 square miles... bottom line, blame Germany, blame the Soviet Union, but Poland didn't attack any one and got screw at the beginning of the war and got screw at the end of the war ...
Poland's eastern border was moved westward and, as a result, Poland lost more than 70,000 square miles of territory to the Soviet Union. Poland was compensated, however, with German territory from the provinces of Silesia, Pomerania, and the southern part of East Prussia; her western border (as determined at the July 1945 Potsdam conference) would run along the line of the Oder and Neisse Rivers. Thus, Poland received more than 40,000 square miles of territory from Germany, including Silesian coal mines and a Baltic Sea coastline. This territorial shift of Polish borders moved the country decisively westward, closer to the heart of Europe. Nevertheless, Poland emerged from World War II slightly reduced in size from its 1939 boundaries.
Link to Map of Poland in 1945