Ancient armies were not *stupid*. Their tactics balance their goals, their constraints, their information available, and their resources available...just like ours do today.

Medieval tactics were generally built around horses as the kings of the battlefield. Infantry formations were designed to protect from fast-moving cavalry attacks (pikemen, for example). When horses and archers were superseded by more-deadly artillery and machine guns and tanks, the infantry formations changed from massed formations to dispersed formations. 

It's a wonderful thought that a small unit could peel off a larger formation to take advantage of some weakness, but it rarely worked in practice: Small infantry units would be hacked apart by much-faster cavalry, decimated by arrows, and finally rolled-over by a larger infantry formation. Mass was not *power* - mass was *survivability*. Mass and maneuver together were power. 

You can fight a medieval battle based on *mobility* and *surprise* and *economy of force* over the more common *mass* (look up [Jomini](https://en.wikipedia.org/wiki/Antoine-Henri_Jomini) for the *principles of war*). Usually this means sowing confusion among the enemy, using deception to mislead the enemy generals, and using a shorter *decision cycle* than the enemy in order to act/react faster than they can. However, that kind of fight requires greater training and preparation...and one of the key constraints of most combatants is the cost of those preparations.

Historically, if you can afford to pay a smaller force to train for months, then you can also afford to pay a much larger less-trained force. Recall that the biggest killer in medieval wars was disease, not combat. Also surprise can be very hard to achieve, especially if enemy spies see your army training. The smaller force tends to have less capacity to absorb the unexpected. So it's not surprising that many leaders chose to go for *mass* over more complex strategies. *Simplicity* in an era of slow and unreliable communications can also be a big advantage.

Generally, most of medieval Europe was too impoverished and squalid to be worth the monumental cost of a Big-Empire campaign of invasion and conquest. Instead, think about small campaigns to influence succession, or to force a neighbor to part with a barony or two. Different combatants, whether ancient or modern, may have different *political goals* for the conflict...and while both may want to extend their *influence*, that influence might not involve conquest at all. If that's confusing, then you just haven't read enough Shakespeare.