[Malc]

Can't say I've personally came across this, but I guess it can happen as each snake is an individual and who can tell what goes on in the genetic make up whilst the snake is developing. Normally, even with slow starters, they soon catch up to their siblings size when eating the same amount of food. But as you say, for 18 months there is a striking difference !

 

[Malc]

The more us humans mix and match genes to produce more designer patterns or traits the more chances there are of discovering mutant trends, shark nose (under developed lower jaws), the corkscrewing with spiders, larger eyes in some super forms to name a few. It could be that something in the genetics has caused some of the issues mentioned above. Not all supper lessers are bug eyed snakes, not all spiders produce spiders with cork screwing, and maybe the combo here produces metabolism issues which don't always occur in every hatchling, and you have just been unlucky ?
As mentioned above, see how it progresses - weight the defrosted food item, and weigh the snake at regular weekly times, this way you'll get an average as the snake may weigh a few grams lighter after defecating. This way over time you can get some idea of the efficiency of the snake to convert food into growth.

 

[Malc]

I recall watching a video some years back where a young female royal had previously laid a clutch of three or four eggs having been paired with a male of the same gene, and all of which produced hatchlings with the same deformity, which sadly was so bad all had to be destroyed ( I think it was missing lower jaws and kinking). But the keeper then bred the same female to a different male, and the same male to a new female to see if the issue was either a: an incubation problem, b: a problem with the original male, or c: a problem with the original female or d: a genetics issue. The result of both clutches was the same defect was seen in all hatchlings, and both clutches had to be destroyed.

Now not saying this is the case in the OP snake, but it could be that the problem is a genetic one, but one that is local to the parents rather than being a global issue. It would be interesting to see how the snake progresses and if it does reach the normally accepted weight and length for a Royal.