Yes and no. Some drugs are known to have definite site specific enhancement by direct action, PGF-2 alpha, HGH, Winstrol and some argue even Testo-prop and Testo-suspension.
The rest has an effect due to the volumizing effect of the oil base of the injected substance. It physically takes up space when injected into the muscle belly, the oil disperses between the fibres, increasing the "resting" volume of the muscle. When trained and the muscle group gets a "pump" due to increased arterial flow, it can, depending on the intensity of the pump, stretch the fascia sheath around the muscle to the extent that it physically at microscopic level tears, thus leading to new "growth space".
But you must have a good knowledge of muscle anatomy before intending to inject site-specific - very, very dangerous if you don't know what you are doing.
Knowledge is power!
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