Tren & lower serum IGF-I increases muscle size greater than (>) Test & higher serum IGF-I†.
Test & higher serum IGF-I increases total body size > Tren & lower serum IGF-I.
On an equimolar basis (equivalent per-mg), of course.
Serum IGF-I is a measure of the liver-secreted circulating IGF-I in the blood compartment. This mitogen increases cell division leading to, e.g., growth of the bones, connective tissues, basically all tissues. It makes us larger humans.
Tren increases the activity of autocrine/paracrine IGF-I isoforms (IGF-IEa; IGF-IEc, also known as MGF) in the muscle compartment. This means that the satellite cell response (reflecting the process of hypertrophy) to these isoforms is greatly increased, "doing more with less."
In the muscular compartment, autocrine/paracrine IGF-IEa is increased by rhGH, and IGF-IEc by lifting weights.
†: Note that muscle size here refers to the myofibril diameter, the contractile units, rather than, e.g., the circumference or volume of the upper arms. The former refers specifically to myofibrilar hypertrophy of muscle fibers, that it can be argued, is overly myopic, because we also care about cell swelling and sarcoplasmic hypertrophy, glycogen, etc.