During WWII the US mobilized approximately 8.2 million men under arms at it's peak mobilization (mid 1944). US population at the time was about 140 million, so we fielded an army of approxiamately 5.7% of the population. Germany fielded an army at peak mobilization of about 8.5 million men under arms, but only about half were German and the remainder were organized from sympathizers in conquered countries. Germany's population before the war was roughly 92 million (including regions like Austria which joined Germany without violence), so it mobilized >4.6% of its population at any time. However, because German casualties were an order of magnitude higher than US the actual percentage of the population which served under arms was much higher. Losses in troops far exceeded Germanies capacity to replace them - especially on the Eastern front. Both sides did not have the economic capacity to significantly increases its fielded forces, though with the US this is arguable since the US was not forced to quite the level of national military aestheticism of German (only the South during the civil war has ever been forced to such extremes). However, it is questionable whether high productivity can be maintained in the face of such chilling deprivations.
Western medieval economies are constrained by the fact that 20-30 peasants are needed to produce the excess food for a single non-farmer. In order to even have an army, the majority of non-farmers have to be carpenters, masons, smiths, plowwrights, teamsters, etc., and even the whole of the government service can't be military - you need law enforcement officials, tax collectors, scribes, and so forth as well. As a result, any attempt to mobilize more than a tiny fraction of the population leads to immediate bankruptcy, and wide spread starvation if the mobilization is of any length at all.
Interested parties may note that most third world countries are not far beyond substitance farming systems, and that many of these countries are dependent on foriegn food aid also maintain standing armies of much greater than >1% of the population. But I don't want to argue the modern relavancy of that, just point out its historical invariance.
On the subject of the Roman armies, I agree that they also were probably < 1% of the total population. I disagree that they were oriented toward the 'occassional small expansion'. Rome was a very restless power, in part because I believe it was maintaining a standing army above its economic capacity to support. This lead to a great deal of adventurism because the army could only support itself by plundering foreign soil. Similar problems can be seen during Europe's 30 years war with mercenary forces that needed to move about in order to stay fed and foddered, and probably every other army which 'lived off the land' (I'm not familiar with the details but if I had to guess Monguls. Anyone know the answer to that?)
It's hard to know what to make of accounts like Benin. I personally find the idea that Benin has a standing professional army of > 3% of its population very hard to believe. This is similar to the Greek statement that the Persian army numbered 1,000,000. I'm much more inclined to believe that that was an accounting of the number of able bodied men who owned weapons, or of the size of militia, or simply the men of a certain age, or what not. I don't believe that 3% of the population had no other job but the protection of the rest, and I certainly don't believe Benin could have put 100,000+ men in the field at any point in the country. The logistic difficulty of putting 100,000 non-producer into a small area for an extended period is even today pretty daunting. But, who knows. Different methods of agriculture create different rules of war, so I guess anything is possible. However, to err on the safe side I'd assemble my rules from things we have pretty good numbers for (Europe, China, etc.)
