If a 9 foot tall giant weighing around 800 pounds fully armored with ten times the strength of a human (Completely proportional hand-wave that), beyond just scaling the mace up with him would their be any benefit to increasing its size further?
Yes, increasing the size/weight would make give it more force in a strike as well as a larger hitting area. So if fighting multiple opponents it would crush more in a blow. A weapons mass has a direct correlation to it's striking force. It will break through a bigger barrier and make it harder to parry.
I have several Polynesian weapons both battle ready and ornamental, the ornaments a stronger man than myself could parry with a machete. The real ones are so heavy that they can only be dodged, not parried.
An iron mace ten times heavier than a normal one would smash through a shield and whatever was behind it through it's momentum. In much the same way that a heavy truck needs stronger brakes to stop it than a car.
With a mace it's going to have even more force and momentum than an iron bar for example, it has more weight on the end rather than balanced weight throughout. This is what gives maces (and softball bats) such a big hit compared to other weapons of similar size.
Increasing the size, and thus the weight ratio in proportion to the giant is a two edged sword; the weapon swings harder, anything in the way is in more trouble than it would have been otherwise - the weapon swings harder, anyone swinging it will have more trouble stopping it going full circle than they would otherwise. So a mace of greater than human scale (so it's bigger than it would be for an 800 pound human) will do more damage but it will also be much harder to control. As to a qualitative estimate of the damage it can do that really depends on far too many factors to give a definitive answer.
Do you need a mace?
I'll assume that Christopher Void wants a mace, but for other readers let's consider this question. A mace is primarily a one handed weapon to be used against armored opponents. One handed weapons are used because the other hand is probably holding a shield or the reins of a horse. If there are no mounts or chariots large enough to carry and the giant wears armor strong enough that a shield is unnecessary then skip the mace and go to two handed weapons.
Mace size depends on the target as much as the user. Will giant be fighting other giants his own size? How much armor will his targets have?
If the enemy is a stationary target such as a wall or portcullis then the giant's aim isn't as important as smashing through the defenses faster than the besieged defenders can get the boiling oil ready.
Suppose that the enemy has giants in armor, then making your mace bigger than scale makes the mace slower and more cumbersome than the enemy's weapon.
If the enemy has only human size targets making the mace larger for damage purposes is wasteful in that it taxes the giant's speed and endurance. But there two advantages for making the mace larger for 'target acquisition'.
Consider those big plastic bats that we give small children. Because of the larger diameter it is easier for a child to hit a ball. At the same time the bat's larger diameter increases the surface area in contact with objects that shouldn't be hit. If the giant has 10 times the strength of a human, and a mace hit is considered lethal, there is no point in making the target ten times more dead, but increasing the size of the mace head to make it more likely to hit the target has a definite benefit (for the giant).
An extreme case would be having giants fight little faeries. The most effective mace would look like a badminton racket. The wide head increases the accuracy. If the face is perforated or an open weave of wire then air resistance is lessened the giant doesn't tire as quickly. If needed the racket could be turned 90 degrees and the edge could be used as a traditional mace, or axe if sharpened.
Historically maces are one handed weapons. The handle length is a compromise between reach, control and shaft strength. As the handle gets longer, the mace head has to get lighter for the user to maintain control with one hand. If the handle is "too long" it will break on the edge of a shield. When a giant has a mace and the target is only human size then the length of the mace handle can be increased because the mass can be decreased and yet still be lethal.
Go through your garage, basement, hardware store or sporting goods store and look at what items could be used one-handed on a smaller target. Will your potential weapon reach the enemy while keeping you out of their reach? Will your weapon be lethal to a target of that size? Will your weapon defeat their armor and shield?
Well if medieval mace is 1-1.5kg and the dude is cca. 3m tall, then I would say that making the mace 3* (3 - 4.5kg - sledge hammer) up to 6* (6 - 9kg) heavier should not be a problem for him. But the main advantage is that the giant can have a longer handle, giving him good leverage.
You could even give him a flail or a meteor hammer to make it longer. At that point you have an anti-siege weapon palisade busting soldier. (But giving this giant a sling is probably the best option)
I remember finding something on historical greatswords a while back. Pretty interesting, and relevant here because you're talking about giving your giant a "great-mace." I was surprised swords that make Cloud from FF7's monstrosity look small were a real thing - 7 foot sword? Real thing. Who knew? They were used to destroy pike formations and mounted units (don't quote me) and the knight had to have help picking it up or carrying it from place to place. Kind of like the medieval version of a crew-serve weapon (like a mounted machine gun in modern warfare). I couldn't find the vid that talked about this, but did find this one talking about how to carry a very large sword.
Followed the suggested vids to another one which does a good at explaining the uses and limitations of different kinds of weapons: sword vs axe vs mace. Looks like the mace was kind of the poor man's weapon with a lot of disadvantages. However, it's good at crushing through armor - especially chainmail. For your giant, he might want to have this to kill a normal sized armored knight or his mount in one blow, or for fighting other heavily armored giants. If there are a lot of big monsters in your setting, he could use his mace to deal with them and have a long knife or something on his hip for dealing with people.
Something to consider on how he might have to fight with a big weapon like this is how it throws you around. So I've tossed Scottish hammers before (fun, recommend) and if you don't let go at the right time the momentum in the the 22lb ball will physically pick up a 200+lb person (me) off the ground and send them flying through the air. If this thing is proportionally similar for your giant (typical medieval mace being much smaller), he'll almost need to dance with the weapon using his body as a counterbalance to use it properly.
Do you need a human like giant?
Why not have elephants in your world be intelligent beings, requiring only a small increase in intelligence if any, and also capable of communicating in human languages. Thus some elephants might become warriors in human wars, believing in one side's ideology or being hired as mercenaries, etc.
You may think that historical war elephants only carried howdahs full of archers on their back, but some were armed with oversized weapons and attacked enemy soldiers with them.
For example, I once read of a war elephant that grabbed an enemy standard bearer, threw him high in the air, and impaled him on a sword attached to its tusk as he fell back down to earth. Some war elephants had long iron chains or swords attached to their trunks and used them on enemy soldiers. Or an elephant could simply grab an enemy soldier and use his body as a club or mace to strike down other enemy soldiers.
There are tools with long handles and blades that are swung to manually cut down weeds. An elephant could use a much m larger version to cut down human soldiers.
Or imagine elephants throwing tree trunks at enemy phalanxes to break up their formations.
imagine a repeating crossbow where turning a crank pulls back the bow so a bolt from a magazine automatically falls into place and the bow fires, and then the action repeats. Suppose that an elephant is trained to turn the crank with its trunk much harder and faster than a human could, making it shoot as fast as a Gatling gun.