The savings/greenness factor comes from 2 places:
1 A 5 lbs roll of plastic welding wire comes with a wire tie and a tag with a bar code and a few dozen words (wieght < 1 0z) and takes up maybe a cubic foot. 5 pounds of plastic toys comes to roughly 15 items each of which ships with a cardboard hanger(brightly printed) and a plastic protective bubble (say 1.1 oz) and occupies .5 cubic feet. The 15 items can be assumed to share a single shipping box (3 lbs and 7.5 cubic feet) thus the total for pre-manufactured objects is an extra 4 lbs of trash plus whatever losses are incurred by moving around all those extra (6.5) cubic feet (big trucks vs little trucks).
2 It is unlikely that your Chinese toy factory has it's own plastic plant nearby. More likely large amounts of plastic are made at point X, shipped to point Y, where they are made into toys which are shipped to point Z. Shipping plastic from X to Z to be made into toys on site removes a step.
It is also worth noting that transportation (and plastic) are currently cheap because the market treats their feedstock (petroleum) as infinitely available rather than limited resource. Changes in that situation are likely in the offing. Also local use of plastic would make more local, small scale manufacture of plastic more competitive. Newer, local plants are more likely to be required to meet greener standerds than older plants with only poor disenfrancised neighbors.