the fascia is a dense membrane that surrounds muscle. It does not expand easily, without serious stretching, which can occur by the massive pumps some prodicts give, or by fascial planing(fucking painful) or diy direct injection of oil based products into the muscle prior to workouts. Doing bis prior to arms causes the muscle tissies to swell, add the pum and it forces the fascia to expand.
Arnold always said it was the pump tht built muscles. Most likely by expanding the fascia, and allowing muscles room to grow.