Also relevant,
“Bodybuilding-type training utilizes very high-volume repetition and set configurations that cause an increase in metabolic substrate stores in the muscle. The addition of glycogen and high-energy phosphates to the cell causes additional water to be stored. This effect, combined with minor accumulations of fat droplets, enzymes relevant to the additional activity, and a moderate increase in contractile proteins, causes the cell volume to increase. It is also quite likely, although not well supported in the literature, that an increase in mitochondria will also contribute to this type of hypertrophy. Indirect support of this concept comes from the fact that the slow-twitch fibers of bodybuilders are the most hypertrophied of all the fiber types measured among weight trainees, or of any athletic group investigated, and that slow-twitch fibers have the highest mitochondrial density of all the fiber types. This indicates that bodybuilding training is sufficiently endurance-based in nature to drive changes in mitochondria-rich slow-twitch muscles and thus may result in increased mitochondrial biogenesis in the relevant fiber types. However, since this type of hypertrophy lacks a significant force-production component, it explains why some individuals with smaller muscle mass can out-lift individuals with much more extensive muscular development derived from bodybuilding.”
– Practical Programming for Strength Training p.92