I will cite fights below, fights that should be searched online.
Weight—muscle, bone fat and fluid—slows motion and causes fatigue
Weight bearing limbs [legs] of large men suffer greater shock when struck while bearing weight. [Marcu Ruas verses Paul Varlens, early UFC] Generally the story of giants in combat sports is a sad one.
An advancing combatant’s weight will amplify the force of defensive thrusts. This concept is used by spear and pike men against charging cavalry.
Gigantism causes an enlarged mesomorphic jaw with the jaw being the principal vector for shocking the cervical spine and breaking communication between the brain and body.
Height exposes that jaw, as well as the vital throat.
Elongation of the spin in tall primates reduces load bearing and increases spinal injuries.
Elongated bodies react less quickly to the will of the fighter and cause a reaction lag.
Overall real giants come in two types typified by Hung Man Cho and Bob Sapp who fought each other, as well as numerous normal size people. Generally speaking the elongated Hung Man Cho type suffers coordination-reaction problems and structural injuries, while the thicker Sapp type suffers from extreme muscular fatigue and muscular injuries.
Hurling large missiles is a good giant strategy, attested to in the Iliad and suggested by the behavior of large primates.
Strike from a distance with sweeping weapons, as accuracy with thrusting weapons will be retarded and with heavy crushing weapons the second strike will be delayed, as the Lord of the Nazgul found out in grey dismay on the fields of Gondor.
Extremely large fighters that act on solid intelligence are invariable out fought by speedier small men. I coached, at the same time, Cory [6’5” 400 pounds] and Brett [4’10” and 75 pounds. This is the same approximate size difference as suggested by BF in inquiring about fight characteristic between 5.5 and 10 foot tall antagonists. At five times the weight Cory has a size advantage similar to that enjoyed by the Devil in Iron over Conan. Brett forever vexed Cory, bruising his hands into numbness and beating the big man’s sensitive knees. However, this was abetted by Cory being an intelligent, thinking fighter who wished to prevail as a martial artist via skill, not brute force. In their last two meetings at around that size both fights were over quickly, via KO to the giant and disarm by the giant.
The giant’s greatest advantages are found in his weight [which may cause his own demise by walking into a thrust] and his grip, which is something he retains even when compromised. Giants should be much more difficult to disarm and enjoy bonuses for disarming.
As for other weapon strategies, BF suggested the giant armor his legs and kick. The giant better armor his legs as they are not only accessible targets but his weight bracing on them makes them absorb more shock.
The greatest asset a giant character can attain in terms of weaponry, is a proportionately thick shield. Whether or not this shield is large or small in terms of coverage, if it is made proportionately thick, normal weapons will not be able to shiver it. It has been found in real contact sparring and competition that larger men are better served by a shield than smaller men as the shield may be used to translate fighter weight into force.
Conversely, protective gear has been determined to serve the oversized fighter less well as it is not proportionately thick and the large fighter’s bulk tends to expand the armor from the interior, compromising and eliminating such protective aspects of the gear as blade drag, air-braking and padding.
Giants should operate as aggressive stalkers, keeping up pressure on the smaller enemy and drawing a stroke. Big fighters with the patience to let a smaller fighter strike the first blow have the advantage of not being disrupted as much by the stroke as a smaller person and are able to nullify the smaller combatant’s reaction time advantage by getting him to spend it up front. Watch George Foreman against Joe Frazier and Ken Norton and George in his comeback, to see this stalking/counterpunching concept in action.
Kicking on the low line is an excellent tactic for large fighters.
Of course, in a setting that has fantasy giants of 12-20 feet in height, all a giant would need would be a staff made of a tree trunk and armored shins and boots to kick humans and horses around like so many squeaking toys. Such a giant could wipe out a company of men in moments.
Lastly, in early role playing games some weapons were given triple damage ratings against giants. This is generally wrong. Simply adjust thrusting attacks against an oncoming giant for their mass to add to the strike, and disqualify weapons which cannot reach a giant’s vitals [like a dagger] from scoring torso kills.
Of course, against giants, eyes will be prime targets, with the throat second and the groin third.
Finally, giants should never overlook the value of pushing. There is a reason why pushing is against the rules in most sports—because it works. For the importance of pushing see the last video below and imagine that every size increase on the part of the pusher amplifies the effect. A push takes the opponent's initiative away by setting him on his heels or worse, on his rear.
Due to the primate structure:
giants over 6.5 feet have biomechanical lag
giants over 7 feet have structural defects and health issues
giants over 8 feet are critically elongated and suffer from terminal defects
Giagantopithecus is an example of a 9.8 feet tall primate that became massive [1,200 pounds] in order to support its frame. It is postulated that if a primate became bigger than this it may not be able to cool down when overheated. In any event, almost 10 feet—and half as broad—is as big as primates have apparently gotten. I would suggest basing giant races on human freaks and this great ape.
I propose a magic using race of gracile giants ranging from 7 to 9 feet, who engage in sorcery to extend their lives and improve their athletic deficiencies. These giants should domesticate extra large animals to draw sleds and chariots, and, in warm climes, could be expected to favor aquatic environments and sailing vessels.
Enslavement of humans would be a key component of such a giant culture. The recognition that humans have inherent strengths such as biomechanical function and longevity, would engender fear and jealousy and inform a doctrine of human demonization and servitude.
Such giants would favor archery, kite-shaped shields, tridents and slashing swords over other weapon types.
Human Giant crossbreeds may serve as a retrogressive tribe of criminals living a nomadic life. Your super-warriors would come from this mix, where you would get athletic seven-foot specimens.
Old growth forests would be the preferred habitat of a giant, slouching subhuman that would throw rocks, have reeking scent glands, wish to mate with human and giant women forcibly, wield simple—and grotesquely effective—clubs as weapons and be covered with two thick coats of hair. Crude stone knives and spears would be within the realm of possibility.
This guy would be a squatish 9-10 feet tall and run between 900-1300 pounds.
His favorite from of combat would be rending [Grendel was definitely a Wood Giant adapted to living in fens.], with his fingernails possibly equal to a grisly claw. Such a giant would be nocturnal and would never leave forest cover during daylight. I like the Wood Giant as a creature who hibernates during the brief arctic summer and then emerges ravenously hungry. He should be prone to wide ranging rapine as he seeks human and civilized giant mates of the unwilling kind during the arctic period of perpetual night. Let’s give him some nice fangs.
Finally, this should be the giant that kicks with his tree-trunk shin bone.