Differences between Light & Heavy Infantries’
1. By Equipment
Logically speaking, if you to look solely at the equipment. Than light infantry would have lighter gear in comparison to their heavy brethren. But this in actuality flawed for a few reasons:
One, If you were to categorize their combat gear heavy infantry would only have armor, a weapon (or two) and, maybe, a shield while light infantry on other hand can either have the same list or have additional equipment.
Two, take two classic opposites from Ancient Greek times; the Peltast and the Hoplite. Peltasts’ fought as skirmishers, throwing their javelins at the incoming enemy and then quickly evade them to avoid a melee confrontation. So naturally, Peltasts’ wore very little, if any, armor to allow them to run faster and farther. Hoplites, being heavy, tended to wear lots of armor (note: this only went for those who could afford the armor). Again armor is a poor way to define a soldier as either light or heavy for as a hoplites armor got lighter and lighter through improved metallurgy the peltast’s gear progressively gotten heavier and more bulky. In the end, theoretically speaking, the equipment carried by either hoplites or peltasts began weighing, more or less, the same.
2. By Level of Melee intensity
Okay, heavy infantry do tend to last longer in melee battles a lot better light infantry. So can we use melee durability as a defining fact? Bad idea and here’s why. Let’s take the hoplite again, an already predefined heavy infantryman. No matter how much armor a hoplite has he is still considered a heavy infantryman. This means that a hoplite with only a helmet, shield and greaves cannot be classified as a light infantryman because of the simple fact he isn’t as tough as a more heavily armored one. Same goes for a group of archers (classic skirmishers and light infantry) can’t be classified as heavies just because they handle a melee fight better than other archers.
3. By Cost
Cost is probably the worst way to define soldier as either light or heavy. Depending on the overall expenses (training, equipment, maintenance and pay) a light could be more costly than a heavy or reverse or be about the same.
4. By Strategic Roles’
Ah, purpose on the battlefield. That’s the way to go. There is probably no better way to define an infantryman as either light or heavy than by his role on the field. We can also say that since light infantry are highly mobile, flexible and adaptable they can have multiple roles where as heavy are limited to only a few.
Light Infantry Roles: Scout, skirmisher, screener (thinning enemy ranks) shock trooper, fodder, support the line (gap filler), bait, hold the line (defend), assault the line (attack).
Heavy Infantry Roles: Hold the line, Assault the line, and support the line
With this mockup I would use two different kinds of swordsmen (both fully armored) as either light or heavy. Say, one has the traditional sword and shield, and the other just has a zweihander (two-hander) sword. The former I would use as a heavy infantryman for his shield boosts his overall defensive abilities and latter as light infantry for his impressive shock value (albeit he could still be used as a heavy).