just doing low reps is never really going to get enough blood in and stretch the fibres which is all that bodybuilding really is
You aint growing any new fibres, you just need to get as much blood in there to stretch them out
Most peoples best bodypart is the one they get the best pump in
the fibers do not stretch from blood. They grow from "repairing" and adapting to new found stress and strength demand.
However I do believe that blood volume adaptation in the muscle is also a factor but I think that is more to do with intra muscular vascularity and the muscle somehow adapting to increased blood/oxygen demand, but that is not the muscle fiber itself, imo.