The thing that they seem to lack is a purpose. Why did someone upscale humans? If anything I can see someone minimising humans (I think I might use the "hobbitti" as a humaniti offshoot), but what advantage does upscaling humans give that cannot be replaced by a good forklift.
Mostly to give the idea conceptual simplicity. I wanted to present something familiar and change one thing about it such as size. It is a humanoid because I don't want to spend paragraphs describing how many tentacles it has, its compound eyes etc. Basically its a creature like us except its 6 meters tall, some things about it has to change because of this, but I want its society to be relatively familiar.
They are mammals.
They come in two sexes.
They bear their young live.
etc.
Their size is fearsome enough. Imagine a whole squad of these creatures armed with what are essentially vehicle weapons. Blasting away at Imperial Marines. Imagine the Imperium is at war with these creature, landing on planets in their giant war machines. Giant troopers knocking down houses and blasting away with their hand laser cannons. One is more than a match for a single human, but humans outnumber these creatures, are more nimble and have quicker reflexes. The giants are slow; not as slow as they seem though as their long strides carry them further than a human stride, they can get up to speed but can't change direction of motion very easily due to their inertial. To them, humans seem wickedly fast and nimble and hard to catch, and hard to aim at, but they can crush one should the manage to grasp them. Shooting at a human with their firearms is like shooting a house cat or a raccoon, they come apart fairly easily, but unlike such a critter, they can fire back. Getting zapped by such little creatures can be surprisingly painful if the proper armor isn't worn. The Imperium could easily overwhelm the Giants with sheer numbers, but the giants aren't the Imperium's only problem and so PCs and local militia are left to deal with these incursions as best they can.
As to why someone would engineer them.
+ They make formidable soldiers for one.
- They reproduce very slowly.
- They consume alot more resources than humans
+ Resources however are not in short supply, and they aren't confined to one planet so who cares.
I think the most likely reason for their creation if genetically engineered was for sport. You know as Gladiators for instance. The ancient race that created them where very compedative sports enthusiasts and bigger athletes were generally stronger and capable of greater feats of strength. They raided a planet of primitive humans and "improved" upon their physical attributes. These Giants were not stupid however, they picked up technology from this ancient race and the ancient race was not cautious enough in keeping control of them. The giants rebelled and killed their masters taking control of their own destiny. When they gained independence, they already had space travel and inhabited the 6 giant moons of a gas giant orbiting a type F star. Movement between these moons was fairly routine and ordinary. An alien ship was then capture and reverse engineered so that the Giants could then produce jump drives. They have since spread outward and have so far filled a single subsector when they made contact with the Imperium. The Giants have an empire of their own, this empire is more centrally controlled than the Imperium since their territory is smaller, and unlike the Imperium, they make extensive use of slavery. Their society tends to be harsh and brutal and they value strength and strength of arms above all else; something they have learned from their former masters and apply the same philosophy to outsides. this tendency is not racial however, it is more an effect of the way their society is organized as they have no concept of democracy or "sentient rights" The Imperial form of government is the only one they know. There are upper and lower classes within their society, so far it hasn't occured to the lower classes that something better can exist. Rebellions do occur, but they usually result in a change of emperors and ruling classes with the end result essential the same as what they were rebelling against. Perhaps they can learn new ideas from other races, but so far their contact with others has been limited to warfare.
While a major concern for settlers on the frontier, the Giant Empire is so far only an annoyance to the Greater Imperium, and the Imperium has yet to dispatch a task force to deal with this threat, though the wheels are in motion, the Imperium has other concerns.