Poland was an important buffer zone between Germany and the USSR at the end of World War II. When the Allied leaders met in Yalta in 1945, President Roosevelt was very ill and would die soon thereafter, so he was not in the best state to negotiate. Stalin, on the other hand, was ascendant, as the Soviet army by this time was meeting little resistance from what was left of the German army in the east and was advancing rapidly towards Berlin. The Allied forces coming from the west were more bogged down and moving more slowly.
Although Stalin had the upper hand at that moment and although his country had by far sustained the worst losses in the war to that point, the United States nevertheless argued with him over the western border of Poland. Stalin wanted even more of Germany absorbed into Poland than the US could stomach. The Soviets, as it was, were able to incorporate a good chunk of east Germany into Poland, simply ejecting Germans and forcing them to move west. The Germans displaced were unhappy but accepted this, knowing that when the German army was invading the Soviet Union in 1941 it simply murdered most non-combatants in its path.
The USSR wanted as weak a Germany as possible, which meant keeping it small,and as big a buffer as possible between itself and Germany, understandable goals given Germany's savage surprise attack on it a few years earlier. The United States, however, wanted to keep the Russians contained and did not want communism spreading into Western Europe. A smaller, weaker Poland was in western European and American interests.