Mechanical properties of ultra-fine polymeric nanofibers are highly size-dependent and the mechanisms for causality of the size dependency is still not quite clear yet. In this work, we investigate the origins of this size dependency at the atomistic level. By using molecular dynamics (MD) framework, two fabrication methods are utilized in this study to prepare non-drawn and hot-drawn fibers, in order to investigate the two distinguished mechanisms for nanofiber size dependency. One is rooted in the effect of surface chains which is intrinsic to low dimensional materials and the other is the chain alignment induced by drawing during fiber fabrication process. Our results show that the size dependency is not chiefly attributed to the effect of surface polymeric chains, but rather strongly relates to the chain alignment in nanofiber's microstructure. The atomistic study shows that non-drawn nanofibers have a dense core covered by a less dense shell layer, but interestingly, our investigations reveal that the core-shell structure itself will not result in the remarkable increase of modulus and strength when diameter of the fiber drops down. By testing the hot-drawn fibers, we found the size dependency mainly originates from the chain alignment. When fiber is formed by hot-drawing, higher drawing ratio leads to thinner fiber and more aligned chains in the microstructure. A three-fold increase of modulus and strength is observed when the chain orientation parameter of the hot-drawn nanofiber increases from 0.1491 to 0.3375 while the diameter of the fiber drops from 19 to 10 nm. By examining the energy evolution associated with the bond lengths, bond angles and dihedral angles at the atomistic scale, our results show that the dihedral angles play a vital role in the size-effect. Our results also show the mechanical response of fibers are affected by changing the temperature, additionally, the orientation-induced crystallization is monitored on hot-drawn fibers at 300 K, which makes the fibers become stiffer but less ductile.