Lifting weights will cause muscles to enlarge. This is primarily due to

This happens due to protein synthesis and muscle breakdown.

Explanation:

Muscle is made of fibers, and these, at the same time, are made of microfibers called proteins, when a person makes an effort to which he is not accustomed, these microfibers break, thus creating microcracks, after a time of rest, these microcracks regenerate and grow to avoid breaking with the same effort.

Because microfibers constantly break and regenerate, the muscle grows larger.