Maybe I'm beating a dead horse here but it just doesn't make sense. The manual says forests are ideal terrain for light infantry since heavy infantry is at "a huge disadvantage". Does a heavy infantry unit completely wiping out a light infantry unit of the same tier while suffering about 20% casualties sound like it is at a huge disadvantage? Light infantry should be able to defeat other unit types in the forest or else there is no reason to use them at all.
Developers are not immune to mistakes. Balance is just isn't there in this game, and this is just minor problem among a others.