I know that new muscle cells are made when actin fibers are disrupted. My hypothesis is that muscle mass is limited, from birth, and that nutrients (protein) are only directed to grow muscle when those muscles are actually used. Is this hypothesis true?
First, it may not be true that you have to actually damage existing muscle fibres to promote muscle growth, see http://jeb.biologists.org/content/214/4/674.full
I do not find papers on the optimality of muscle remodelling, but maybe you'll be interested by a comparison with bone remodelling. Bone faces indeed similar constraints as muscle: it has to be sufficiently strong to resist the forces that we face in everyday life. Astronauts e.g. have paralleled bone and muscle mass decrease if they don't train during flights.
Bone remodelling now is not just reinforcing the bone where it is needed: it also involves bone resorption in places where there is less mechanical stress. E.g, people having a bone implant may lose bone because the implant bears more stress than the implant ! See this paper. Why decrease the bone mass? Well, this is a matter of optimality: just as in engineering, you want the lightest structure that will be able to perform its function. See this thesis and references therein.
Muscle has every reason to perform equivalently. You want each muscle to grow according to its usage in your usual activities, so you need to regulate growth and loss. This is likely to be more important than limiting protein demand—at least as long as food is abundant enough.
A personal anecdote to finish: a friend of mine was a hardworking racing cyclist. He once wanted to take part in a multisport raid, involving some hill running—and couldn't run the way down, because of his overgrown thighs. A little of diversity in his personal training would have allowed him a better balance in muscular weight!
Muscles are developed as required by the surrounding environment. This means your body is constantly physically stimulated to grow muscles according to the external forces it experiences (e.g. gravity).
When you train your body, e.g. with weights, you are applying an unusual amount of pressure to your muscle cells, damaging them in the process. In the process of healing your muscle cells, your body will strengthen them in order to adapt to the new physical stimuli, but only when a vast amount of required elements through nutrition is available (proteins).
Quite an impressive feature, right. :-)
I know this from researching personal training/weight training.