This country has very low mortality rates and basic medicine, it is also very well-connected with a strong government and homogeneous culture and people. The country is self-providing of all resources. The army doesn't engage in massive battles and is mostly border guard and garrison.
The events are set in a late renaissance-like world(not specific to Europe), minus the gunpowder. The country has political feuds growing with a nearby nation but doesn't have enough troops to challenge them military, why would that be?
They haven't had any major war for decades, the population is about 100 million, while their military force is 250k in total (the age demographic of the population is evenly mixed). Consider forced conscription unavailable.
some extra clarification 250k is small because of its size, (something like the former USSR, but the land is more of the weather of western Europe) there are thousands of castles bordering other nations and a huge navy that protects the trade routes, the number also includes the police force.
Note: i keep seeing people using the "The events are set in a late Renaissance-like world" as a reason to compare it to Europe. i used that term for the lack of a better term to symbolize the technology, the weaponry, the advancement in sailing, governmental structures of that time, and chain of command.I am mainly referring to the time period. also, the population count shouldn't matter, what matters is that i am saying that in this case it makes sense that they can have a 100 million and that a 250k army is small. what would be reasons that they wouldn't be able to mobilize further
extra clarification: consider transportation across the country to not be a problem, there exist a series of channels that interconnect all parts of the land (dosent matter how they were built) they can cut travel times by half.