I am no expert but I will throw in a guesstimate.

I 7ft tall 400 lbs man has more blood than a 5 ft 100 lbs man. Seems obvious.

However, I believe what matters is the concentration of testosterone in the blood. A testosterone receptor in a muscle doesn't care how much total test exists in a person's body. It cares about how much test is swishing around it at any given moment. The concentration of testosterone in the blood in the general vicinity of the receptor is what matters.

A testosterone receptor in the bicep doesn't care how much testosterone is located in the calve muscle. It only matters how much testosterone is around it at the moment. The higher the concentration of testosterone around the receptor, the better chance of the testosterone binding to the receptor.

Thus the concentration of testosterone in the blood is what is most important.

I of course could be totally wrong.