Muscles increase in volume after long-term weight training.
Since they are made of individual fibres, muscles can theoretically increase in volume either (1) the number of fibres increases, this is called hyperplasia, or (2) the volume of each muscle fibre increases , this is called hypertrophy.
Either of these processes involve an increase in the protein content of the whole muscle, which is known as hypertrophy.
Hyperplasia
Researchers have observed signs of fibre splitting after a very strenuous programs of voluntary weight training, but to date we have no real solid indications that long term weight training causes increases in muscle fibre number. Some research suggests that weight training does not cause hyperplasia in humans, as indicated by comparisons of untrained people and bodybuilders that show similar numbers of muscle fibres in the muscles of both populations. Therefore it seems unlikely that hyperplasia contributes meaningfully to whole muscle growth after weight training.
Hypertrophy
Increases in protein content, and therefore the volume of individual muscle fibres can occur because either they increase in diameter or cross-sectional area, or they increase in length.
Many studies in humans have shown that muscle fibre fascicle length (fascicles are bundles of muscle fibres) increases after long term weight training. This happens particularly often when the training program involves eccentric-only contractions, or when peak contraction of the exercise occurs at long muscle lengths.
Similarly, the diameter of individual fibres also increases after long term weight training. Increases in diameter are sometimes greater in type 2 fibres, likely because type 1 are more commonly, but not always linked with lower threshold motor units, and generally only the higher threshold motor units increase in size after weight training.