Danke, Oli. But 20k is fewer line troops than Napoleon required of them, correct? In 1810, for example, Saxony's field army was about 25k Infantry, about 6k Cavalry, and came to something like 32,000, if I'm not mistaken.

(And - I'm no expert on this, admittedly - didn't the Allies end up taking only about 16k Saxons into their army in late 1813- early 1814? The rest stayed home, correct?)

The Landwehr referred to here, was comparable to the Bürgergarde under the Rheinbund, correct? So that's comparable in size to that body during their alliance with France.

Men are men, regardless whether they are line or Landwehr, and where they stay: they have to be taken from the population and supplied with clothing and weapons and food. The only difference was (I am not 100% sure for the Saxon army, though) that at the end of the war Landwehr men and volunteers were sent home, whereas the men of the line troops were supposed to serve for longer.

If the Saxon contingent in fact did not reach 40.000 men in 1814, due to problems of finding enough recruits and equipment, it doesn't mean Saxony wasn't still required to provide them. I am not aware of any official reduction of the required number. Saxony was just lucky the campaign ended in the first days of April 1814.

Even if the Saxon army had been stronger in 1810, Saxony was required to provide only 20.000 men for the Rheinbund, the rest was the Saxon king's private pleasure:

http://www.napoleon-online.de/armee_rheinbundstaaten.html

On a related topic, are you aware of any texts or documents that detail the comparative forced contributions, Franzosensteuer, and so on, under the French, vis-a-vis under the Allies in 1813-15?

Unfortunately not.