Well, I don't do too much replacing of cav with FMs, it's just that I have always brought along 2 family members per institutionalised fullstack army, a long standing army composition rule thing. After all, the generals always fought from the front in good ol' days. I just want more survivability, more than that fella across the battlefield from me with them wavy plumes and big bodyguard. And I just can't afford the time to accrue influence, it takes reaaaaally long, and there isn't even the guarantee that he will get 10 influence always, by a certain period of time.

Ah well. I've found out what I needed to know. I always thought command stars influenced the sizes, seeing some link between all the 9-star and 10-star generals I fought and their huge bodyguards, and seeing how only noob AI family members newly-come-of-age had 40 bodyguards, and not for long...