I was watching a history video that mentioned that the Polish-Lithuanian Commonwealth was the largest European nation in the 17th century. This fact surprised me, as I had previously done a bit of research into Polish military history and had found no major military conquests. They had noteworthy defensive victories against the Teutonic Knights, Mongols, and Ottomans, but never had a period of military supremacy like Roman under Ceasar or France under Charlemagne and Napoleon.
Absent major military conquests, how did the Polish-Lithuanian Commonwealth grow so large? This proved somewhat difficult to find. For the first few hundred years of its existence, the Kingdom (sometimes Duchy) of Poland waged numerous wars with its neighbors and grew slowly into a regional power. There was no great conquest in this period, but Poland proved consistently successful in taking small gains and holding them.
Finally, I was able to find an event that considerably increased the territory of both pre-Commonwealth Poland and Lithuania: the conquest of the Kingdom of Galicia–Volhynia. Poland and Lithuania teamed up against the Kingdom of Galicia–Volhynia during a succession crisis. When they unified, much of the Polish-Lithuanian Commonwealth territory was had been previously Galicia–Volhynian land.
Light Pink areas were added during or after the war with Galicia–Volhynia Source
My afternoon of research ended up painting a clear picture: Poland started as a small state that slowly absorbed its neighbors for hundreds of years. Then it conquered one of its largest neighbors. Finally, it unified with another neighbor, making it the largest nation in 17th century Europe.