I agree 500k is overkill, however their empire in game was very large, but still overkill. Even the HRE having 200K, in 1200!? Just a tad overkill to be sure.
However, 20k seems a bit low too. Remember, the first crusaders that took jerusalem in 1099 with 60,000 men. A strong HRE with 100,000 wouldnt be impossible IMO. However, England a place like england should be more like 20k to 40k.
It's easy to muster people when "God wills it", but I doubt the King of Eng or even Emperor could muster tens of thousands of men in the 11th and 12th centuries simply to assist his ducal ally in putting down a rebellious count or claimant.
At least I never read about that in a history book.
In CK2 I attacked Flanders as Lower Lorraine and his marriage to Madame Doukas brought 14k Greek soldiers into the North Sea to defend this lowly duke against my claims. Luckily my holy orders were able to turn them into fertilizer for the tulips, but it seems a bit ridiculous for that kind of commitment over a petty ducal squabble.