I've been staying out of this one for a bit, but I think it is time to jump in
1) Not sure where most of the numbers for historical reference are from. In the American Civil War, total Union casualties were 360,322; Confederate losses were estimated at 258,000 (multiple references for this one, but all are based on Livermore's Casualties and Losses in the Civil War). Total combatants of both sides varied in number, but were around 1.2 million at any one time. Keep in mind that the actual number of men in uniform during the war (i.e., who served for a while then were demobilized/went home at the end of their service) was closer to 4-5 million.
2) Over 400,000 EMPIRE troops served in the Boer War (most common number cited, from UK Official History is 448,435; total Empire deaths slightly over 21,000). While the British provided the leadership and many of the units, the troops came from all over the British Empire.
3) The US Army was very small throughout the 19th Century, and this was on purpose. While the active Army was only about 28-30k at most (usually in the low 20's, since it was not considered a good place to work

), the bulk of the officer corps (well over 90%) were West Point graduates--but there were relatively few officers vice soldiers (think more of a 1/50 ratio than a 1/10). The US Navy tended to be much more modern and well equipped in contrast. Again, this is on purpose. In the US Constitution, it clearly states "The Power to make War, and to raise Armies and maintain a Navy" belong to Congress. In effect, this means that the US would have a bare minimum of a standing army (backed by, in effect, the entire manpower of the nation in wartime) but would be REQUIRED to have a standing navy. As an active duty US Army officer working in the Pentagon, I can tell you that this still applies--the Army has to go to Congress on an annual basis to get money, while the Navy does not (every two years for them). Why? Because Congress has to vote to KEEP A STANDING ARMY for another year (aka, the Annual Defense Budget). So I think the mobilization concept for Vicky is perfect for the USA at least.
4) While not a Prussia/Imperial Germany expert, the bulk of Germany's army was made of long term conscripts as well as professionals. The idea was to rotate and train as much manpower of the nation as possible for war--lets be honest, the Germans live in a bad neighborhood in the 19th century. Consequently, they should be on the far end of the spectrum for military expenditures and manpower. Check out
http://www.replications.com/8KR/krstats.htm for an excellent example of where all of Germany's money was going in 1914. Nearly 50% was going to defense. Again, if a player wants to maintain a 100 division German army in Vicky, they can do so, just don't call it 'historical' based on some view of Imperial Germany in 1914-1918. No country in the era could keep a huge standing army at 100% in peacetime. Their economy would collapse, or worse, they would end up like Russia--huge army, poorly equipped, with bad morale, one disaster away from revolution (read 1905 and 1917).
Sorry about the long post. I just get tired of people quoting numbers without any kind of reference or placing the statistics into some sort of framework.
