My take on the leviathans based on what we've been shown or told so far: from the way they're they've described in this book, the leviathans clearly used to be people who've mutated from augmentations. They still have vestigial eyes and faces, and some try to talk. The one Din sees near the end of the book is gray, which we are repeatedly reminded is the cast that people's skin takes as they receive augmentations.

Various characters reference multiple classes or castes of people, the workers who do heavy labor and the higher-ups in the military ranks, who grow to enormous sizes due to their augmentations. The workers ("cracklers") get retired when they grow too large, but the wording used in that part is vague enough that we don't know for sure what really happens. Even people with certain types of smaller augmentations, like nasal enhancements, have noticeably abnormal growth and alterations in physical appearance.

The uppermost ranks of society are seen less outside the privileged classes, so them diverging significantly from a normal human appearance is only an issue if they are regularly seen in public and/or need to travel. In the scene where Din and Fayazi visit the family bathhouse, we learn that Fayazi's father decided he was at his limit for augmentations because he was well aware of this, and there's some brief but crucial info shared about conzulates. Most characters in this novel have some degree of security clearance, so we don't know how much of this is common knowledge among the masses. Therefore, the laws regulating the amount of augmentations in the lower classes are to prevent the general public from reaching the tipping point where someone mutates into a leviathan in the public eye and causes mass riots.

We do have a good idea of why this is perpetuated—defense against leviathans is the glue that keeps the empire together, and regular citizens would rise up if they found out their ruling class was literally mass-slaughtering them on an annual basis—so the main question to be solved in future books is, how did this augmentation/leviathan mutation cycle originally start?