Dominions 5 uses anatomical locations to determine where an adversary was struck. Certain hits can cause afflictions, such as losing a limb. If someone loses a limb, it will be the one which was hit. This can be an arm, leg, or head. Losing your head is usually instant death, but this is not always true, for example in the case of undead and certain kinds of regenerating monsters, or those with multiple heads. Other types of afflictions include losing an eye, which can only happen on a head hit.
In order to score a hit on a certain part of the body, however, the attacker must be able to reach it. To score a head hit, attacker size + weapon length must be equal to target size. This requirement is one less to hit the torso and two less to hit the arms. Thus a human (size 2) wielding a mace (length 1) could hit a size-6 creature only in the legs!
Some monsters have their heads located lower than their size would indicate and therefore anyone attacking them will get a bonus to their reach. Lizards and dragons are very low and are two steps easier to hit when striking anything higher than the legs. Most four-footed beings like elephants and wolves are slightly lower than a humanoid would be and are one step easier to reach. Their lower stature does not mean they are more likely to be hit in the head (or any other hard-to-reach location). It is still only a 10% chance, but attackers with shorter weapons might be able to reach and hit them in the head in situations where their size might otherwise prevent it.
Also, some monsters lack certain hit locations. For example, a dragon has no arms and a gelatinous cube has no arms, legs, or head.
A significantly larger unit that attacks a smaller one will have an increased chance of hitting its target in the head and a lower chance of hitting it in the leg (20% head, 10% leg). To be considered “significantly larger” it must either be 2 sizes larger, or 1 size larger and be mounted.