AAS injected directly into the muscle belly DOES increase localizecd growth. This is due to two factors. First, the "on site" of any fast/long acting steroid induces greater localized receptor-site stimulation. This may be due in part to the fact that lagging body parts often have a lower circulatory supply of blood and the steroid/nutrients it carries.
This in turn often relates to inadequate development of vascular and nerve factors in these areas as well. WELL RESPECTED authors state that by introducing a space occupier, muscle fiber and belly fascia was forced to stretch and increase in area. This induced localized growth in turn due to...greater area allowed the hypertrophy of muscle cells and fibers to increase.
Think of it this way: If you put a ballon in a coffee can, you would only be able to blow it up so far. It would simply occupy available space. However, if you created a larger coffee can, you could more easily increase the volumn of the balloon.
Oh...and test prop. is one of the best site injection steroids...and many people do use site-enhancement oil between cycles to maintain the "fillable space".