Although this is going to oversimplify it- I believe it has to cellular proliferation of mitochondria and the cellular mechanism of supporting that proliferation. Muscles are such a unique cell to the body.

Plus the activation of satellite cells is another key factor in this debate.

But if you were to ask me what my best guess is -

The curve in which load x time under tension, and the graduating curve of required cellular energy output .. Not sure how to accurately represent this theory.