Pangur Bán said:
Such is the logic of dynasticism.
Though what really made Poland-Lithuania into one country was not the Union of Krewo in 1385, but rather the Union of Lublin.
The Union of Lublin of 1569 established a common Polish-Lithuanian parliament - the General Sejm (consisting of Chamber of Deputies and Senate). In General Sejm there were originally 120 deputies from Poland and 48 deputies from Lithuania, as well as 113 senators from Poland and 27 senators from Lithuania. Later numbers of deputies changed a bit. For example in years 1632 - 1648 Poland had 138 deputies, Lithuania 48, and Livonia 6 (total 192).
If counting those 192 deputies with use of modern borders: Poland - 103, Ukraine - 35, Belarus - 32, Lithuania - 12, Latvia - 6, Russia - 4.
After the first Partition (1772), there were 123 senators from Poland and 31 from Lithuania, while deputies numbered 177 - 123 from Poland, 44 from Lithuania (compared to 48 before 1772) and 6 from Livonia. But in 1775 the number of deputies from the GDL was again increased to 48.
Treasuries remained separate for both realms of the Commonwealth, though. In the 1570s tax revenue of the Grand Duchy of Lithuania amounted to just around 20% of tax revenue of the Crown of Poland. In period 1587 - 1632 that proportion was a bit less disadvantageous for Lithuania - her annual tax revenue in that period amounted to about 35% of Poland's. But then in years 1632 - 1648 Lithuania's tax revenue again shrank to 22% of Poland's.
Although tax revenues were not only the result of economic potential, but also of economic policies (since in the PLC there existed no constant taxes, but taxes had to be enacted by the parliament and / or by regional councils each time when money was needed), this data still tells us something about the economic potential of Belarus (the core of the GDL) compared to that of Poland. The only year in which the GDL had similar tax revenue as Poland was 1613 - apparently in that year the GDL enacted a lot of taxes, while the Crown didn't (though the PLC was - generally - a tax haven by modern standards).
Of course Poland in that period was enlarged compared to times before the Union of Lublin. Before 1569 the Crown of Poland - excluding Ducal (East) Prussia, Courland and Livonia - had ca. 256,131 km2. As the result of the Union of Lublin, Poland acquired the region of Podlasie (11,507 km2) - located in the Polish-Belarusian borderland - as well as Volhynian and Bratslavian voivodeships (altogether ca. 88,374 km2), and Kievan voivodeship - the size of which was kind of fluent, but at it's peak it was perhaps ca. 200,000 km2 (though much of that were uninhabited wastelands and steppes) - but after the treaty of Andruszowo (1667) and the peace of Grzymułtowski (1686), Kievan voivodeship shrank to ca. 68,953 km2, and remained that large until 1793.
Moreover, in period 1618 - 1667 Poland established yet another voivodeship in the east after conquering the land of Chernigov from Russia (in the same period the land of Smolensk was incorporated to the Grand Duchy, where as well another voivodeship was established).