Is it because I started raising Rue in dry conditions that she is not growing as fast as Kang?
Yes. Absolutely. 100%.
I've seen this over an over with many species. Insects, lizards, snakes, and tortoises.
A few years ago I raised a bunch of South African black throated monitor lizards, Varanus albigularis, on deep damp substrate and gave them all humid hide boxes. I also soaked them in tall tubs 3-4 times a week. They all reached 4 feet (122cm) in their first year eating mostly insects. I raised them dry, as instructed by the books, experts, and websites in the past, and they took 2 or 3 years to reach that size.
Same story for all my tortoise species. Good hydration keeps them healthy and thriving.