1.France had the largest army up until about 1815, then the United Kingdom took over.
2. France was Prussia's largest enemy due to Napoleonic Wars. The army was outdated by the beginning of the Napoleonic Wars and Prussia was defeated by France in the War of the Fourth Coalition.
3. They all kept the countries the same size and didn't allow one to be dominant over the others because Prussia’s reward for its part in France's defeat came at the Congress of Vienna
It regained most of its pre-1806 territory. However, as compensation it picked up some new territory,
including 40% of the Kingdom of Saxony and much of Westphalia and the Rhineland. Prussia stretched
uninterrupted from the Niemen in the east to the Elbe in the west, and
possessed a chain of disconnected territories west of the Elbe which in turn regained their portion of territory back.