Realistically it’s how treaties were signed and how post-1795 things worked out. Poland would gain somewhat of a shape in 1805 when Napoleon made it a puppet, which its size somewhat continued into the Congress of Vienna. Poland was granted autonomous kingdom within the Russian Empire, and its controlled territpry. Lithuania, however, was ignored. Of course Poland would lose all autonomy by 1840 after failed revolutions leading to a massive setback as retaliation. During WW1, the Germans once again set up a puppet Polish state in the general area, while Lithuania did not get the same treatment. During the 1918-1920 Polish Soviet War and Peace Treaties, Poland militarily acquired large amounts of territories, including from the area designated as Lithuania. Since then, their general sizes have not changed significantly, conforming Poland to becoming a larger nation than Lithuania despite the opposite during the Commonwealth era.

TL;DR - Inperial forces and treaties ignored Lithuania and set up the norm that Poland was a state earlier on than Lithuania, hence solidifying their positions

You shouldn't identify the Grand Duchy of Lithuania only with a present Republice of Lithuania. Belarus and wester parts of Ukraine used to be part of this country too, without a doubt we can call them successors of Commonwealth.