Axial length of the eyeball is the distance from the lens to the retina. Elongation is the scientifically accurate reason why anyone has myopia.
It would require precision engineering to be able to grow an eye that was perfectly emmetropic at birth. Instead, it is of approximate dimensions, and feedback during postnatal development tunes the axial length to match the focusing system. Two mechanisms are available: changing the thickness of the choroid, and changing the length of the eyeball itself. 
How it can be reduced
- Wallman J, Winawer J (2004). "Homeostasis of eye growth and the question of myopia". Neuron. 43 (4): 447–68. doi:10.1016/j.neuron.2004.08.008. PMID 15312645.