By the rules there is no bonus specific to this, but the bonus comes from the mechanics as Hithero said. Note the following:
F3 warrior charged by F3 cavalryman has 34% chance of winning against 66% of the cavalryman.
F3 warrior with spear support against F3 cavalry has 50/50 chance of winning
F3 warrior with spear and pike support against F3 cavalry has 60% against 40% of winning combat.
I hope you can see where this is going - the more support you have, the bigger impact it makes for the combat and the greater it nullifies the cavalry bonus.
F4 warrior charged by F3 cavalry has 42% of winning
F4 supported charged by F3 cav has 61% of winning
F4 supported by spear and pike against F3 cav has 72% of winning.
Now if cavalry charged first, then due to priority the infantry can add additional models into the fight after that, which can result in something like 6 attacks vs 2 with higher fight, leading to stuff like 87% winning chances. At the same time Fight Value plays a big difference, as it allows big heroes to almost ignore the pike blocks and still rip through them like it is done in films
There is clearly no need for any more specific rules. It is not stupid at all.