Prussia had a militarist culture thanks to Teutonic heritage, also their leaders were Illustrated Despots during the XVIII century which made them modernize their state in several senses, they earnt more land through careful diplomacy, marriages, military conquest, etc. they also had a natural access to the Baltic sea.
It is not strange at all if you read the history of Prussia, the other German states were too insignificant both in size or population (except perhaps Bavaria or Austria that already had an empire) or were political structures like free cities, archbishopries, merchant republics so it didn’t made much sense for them to expand.