For a while this puzzled me because if you look at the partitioning of Poland between the two dictators, it looks like an even split:But this is deceptive on two accounts. First, the area occupied by Nazi German had a much higher population density:And was inhabited mostly by ethnic Poles while the area occupied by the USSR was predominately Ukrainian and Belarusian:So the area occupied by Germany could be thought of as "Poland Proper" while the area taken by the USSR were more like "colonial possessions". To give an analogy, it would be like if in an alternate history scenario, Germany and Brazil made a pact to go to war with France. Germany launches the initial invasion from the East, captures Paris at which point it's a matter of when, not if France falls and then Brazil invades their colonial possessions in South America and the Caribbean. If Britain declared war on Germany after the initial invasion, while they would protest the Brazilian invasion (as France and Britain did to the USSR in our timeline), they would be hesitant to go to war with Brazil and spread themselves too thin until Germany was defeated and the French mainland was liberated.