I know the difference is partly because Lemmy was a larger platform, and is better known but the growth of Kbin seems suprisingly flat while the community seems to be getting more and more active?
Precisely that. Moreover, Kbin is more fresh than Lemmy (Lemmy seems to have been around for a while longer), and Kbin still lists itself on the website as an early beta.
OTOH, there were already more Lemmy servers to begin with, while there were no third party servers at all iirc when the Reddit migration started.