@Maugreas @Kyujyu I invite you guys to actually read more about the theory of military strategy, particularly about Lanchester's laws, and concentration of force.
The only time when concentration of force can be overridden in an attack is when there are huge area of effect attacks, when there are chokepoints that limit your maximum "surface area", and when there are very distinct unit types. Neither of these conditions hold for the ogres (although the last condition holds for the humans, and their formation makes sense).
If your front line needs reinforcing, it is far more effective to just have the reinforcements there in the first place; you'll whittle down the enemy forces meanwhile, and (unless there are very effective AOE attacks) you'll not take any more damage than usual.
Concentration of force is also relevant in a defense, but the usual circumstance is that the defenders do not know the attacker formation, so they use a weaker front line as a signalling mechanism that calls the stronger forces behind to the attack location. In that case, the front line does not charge at the enemy; their objective is to hold out for as long as possible, collect information on the attackers, and signal the reinforcements to their location (for example, the long front lines during WWI would signal tanks to come to their location to rescue them *while moving backwards*). In no circumstance do we have units waiting idly for an attack, and in no circumstance does a weak group charge at a clearly stronger group; if you're weaker than the enemy, your job is to stall for time until reinforcements come and charging does not give you more time.