I'm not really concerned about the total size of retinues as long as they're reasonable compared to total manpower- the game naturally starts drifting away from realistic troop numbers as soon as it gives players the ability to directly increase troop numbers through buildings rather than having to wait on population growth or social change. But I do agree that retinues should be tied to Crown Authority or some other set of laws. Retinues are such a powerful tool for maintaining a liege's authority that it makes sense many vassals would want to limit them, especially in cases like the HRE.From memory this was at the point where France shifted from feudal to royal (standing) army. Still as a comparison, France at endgame, with Mil Org of average 4, would be throwing up retinue numbers much larger than this.
They probably need to readjust the numbers and tie it in to crown authority more (so MP from demense + a proportion of MP from realm). The HRE shouldn't get 10% of his realms manpower at game start for his own personal army. He struggled to get any troops from much of it at all.
The concept of retinues is ok, its just the calculations of what goes into it that need work.