It has somerthing to do with enemy strength calculation as well, according to the autoresolve stats. I've killed peasant armies before and got nothing, 1 Bodyguard vs 600 rebels... not heroic ? Well it seems it is'nt for this game. However 1 Bodyguard vs 600 HRE of which 300 cavalry, 300 DGK => Heroic Victory. (exagerrated example)

There is a qualitative factor.