In the past decades, the price for single mode fibers for telecommunication decreased from a few hundred dollars per km to under ten dollar per km today. One key contribution to save cost, is the economy of scale. The more volume a producer can manufacture every year, the lower the overhead cost per unit.
It also proved beneficial to increase the individual batch size; the km of fiber drawn from a preform. Many processes need to be done independent of the size of the preform, transportation from storage to draw furnace for example. The cost might increase a little, as the preform gets heavier but that is offset by the number of processes saved.
For example, five 90mm preforms yield the same volume of fiber as a single 150mm preform. While it may take one hour to set up the draw and get fiber draw started for a 90 mm preform, it may take two hours for a 150 mm preform. For the same production volume 3/5th of the setup time is saved.