[Taqwus]

Non-uniform training time might well have a big impact, and seems reasonable -- untrained, generally lightly-equipped irregulars could be recruited and mobilized much faster than those who need to be trained to fight in armor and in close formation, and it'll take even longer if horsemanship or archery plays a part. Even if you have plentiful material resources, training will still take time unless your society's so militarized that your recruiting pool is already trained as a part of their upbringing.
And yeah, demobilizing makes sense. They may be irregular conscripts rather than professional soldiers, and once the threat's over they'd rather go home. Conversely, if there's a nearby threat loyal citizens might even volunteer to protect their homes (e.g. higher chance of militia event).
Combining the above with high maintenance cost for maintaining a standing army would increase the incentive to keep a smaller army followed by rapid raising of militia.

Edited: Incoherent and dropping words today.

[ October 29, 2003, 21:54: Message edited by: Taqwus ]