If you peer into the (un-tendentious) history of much of those lands, you might take a slightly different view of them and their role and importance in Polish history, culture, language, and statehood, beyond just the 20th century... But perhaps more to the point, Poland lost nearly half of its prewar territory, east of the Curzon line. Poland is territorially smaller today than it was before WWII.
It also "received" several million of its own people killed, including the highly educated Jewish community. While we are crunching numbers, let us not forget that loss of human capital matters in economy as well.