Maybe it isn't 100% reversed. After all, how would a being give birth to a child the same size as the parent?
So, perhaps, they develop from an embryo like a human, go through the same or similar stages - small infant, toddler, child, pre-teen, teen, full size adult. Then, about the mid-point in their life cycle, they go through the reverse and begin to shrink and shrivel up. Possibly there are noticeable differences between a 40lb 4 ft. Child and a 40lb 4ft elderly individual.
Although, Mork and Mindy had Mork lay a small egg that grew to ful term and the full size of Jonathan Winters.
Ah, shazbot