I suspect that this is something that arose, over the years, first amongst figure resellers (who tried to come up with "standard" boxed armies based on published army lists), and then among rules writers to provide standard armies for systems that did not have points values (I am thinking of Tactica, etc).
Recently, I started organizing my 15mm Napoleonics for Shako II. 99% of the games played in ODMS using Shako are scenario driven (some referee picks a scenario and sets the forces and terms of deployment and victory conditions, ignoring the standard army lists and tournament rules etc). This is the way I usually like to play wargames. However, I am using the army lists as a helpful rubric (like that?) for deciding how "large" my forces should be. You know, so I don't try to build the whole invasion Leipzig campaign, man for man, in miniature.
My observation is this - those are pretty large armies. For instance, the Prussians (1813-1814 variety) have an army that looks like this:
10x Battalions of Line Infantry
10x Battalions of Landwehr Infantry
5x Battalions of Jaegers/Fusiliers
2x Battalions of Grenadiers/Guards
1x Regiment of Cuirrasier
2x Regiments of Dragoons
1x Regiment of Landwehr Cavalry or Lancers
1x Large Gun Battery
1x Medium Gun Battery
2x demi-Battalions of Skirmishers
That is with 12 figure battalions, and 6 figure regiments. The size of units is flexible, and a battalion could be (reasonably) as few as 6 or as many as 24. Most that I know of are doing between 12 and 16.
Anyway - does that seem kind of large? Almost all of the standard armies in that ruleset are approx 30 units (more or less - the Prussians are more, the British are less, etc), plus artillery and skirmishers.
I know that rulesets like DBA do it smaller, and that Tactica armies could range quite large. But this seems big for a "standard" army.
Please comment below ?