The 30 Years' War (1618-1748) really messed up Germany and subsequent Prussian monarchs really tried to turn Prussia into a Fiscal-Military state. By the time Frederick II came to power in 1740, Prussia had a substantial army with an incredible citizen:soldier ratio (I really can't remember it at the moment). On top of this, previous monarchs had amounted quite a large treasury which Frederick also used to further modernise the armies of Prussia but also the bureaucratic state system within Prussia.

In the War of the Austrian Succession (1740-48, also the first and second Silesian wars for Austria and Prussia), Frederick managed to take and hold Silesia from the Austrians. This, to an extent was really the first sign that Prussia was a 'big player' during the period.

Then in the Seven Years' War (1756-63), Prussia was able to withstand the combined efforts of France, Austria and Russia with the financial (and limited military support) of Britain. Whilst there was a lot more that I could mention, these are a few points which explain or demonstrate Prussia's military expansion during this period.

One last thing that I also think is important during this time, is the system of alliances that formed across the start of the 18th century in order to create a balance of power in Europe. Although this was originally created to contain French hegemony, Prussia was able to ally itself advantageously with nations such as France (war of the Austrian succession) or Britain (seven years war) which allowed it to maintain its militaristic, diplomatic and state wide expansion.

I study history and this question really is a big component of my previous and (hopefully) future research. Hope I've helped!