There can be a tremendous variability in the size of hatchlings, not only from one clutch to another, but even in individuals in a single clutch. Throughout the years, I have speculated on all sorts of causes for this, only to have each one eventually shot down by examples that disproved my theory. So all I can say about it is that "shtuff happens".
I have had entire clutches hatch out that I could not sell because they were way too small and had to be held back and fed up until they looked like normal hatchlings. I have a REALLY pretty Hypo Lavender that was ridiculously small when it hatched, but had an appetite that was unbelievable. I turned him over to my wife and she fed him every two days (whenever possible) because that's the way his metabolism seemed to be geared. If I had tried to take care of him myself, feeding on my typical 5 to 7 day schedule, that animal would have died. Anyway, he is growing pretty steadily now, but still small for his age, but he will eat anytime something is offered. My wife named him "Chuckie Hoover" (Hoover for the vacuum cleaner brand).