- Anatomical cross-sectional area: Often measured as the widest point of a muscle along the muscle's length
- Muscle structure: The length and angle of muscle fascicles (basically just a bundle of muscle) in relation to the line of action of the muscle
- Individual anatomical differences: Specifically, moment arm length, which affects torque at different joints
- Neural drive: How effectively your brain recruits motor units (muscle groups) and how frequently these motor units fire
- Muscle fiber characteristics: Distribution of muscle fiber type, based on type I ("slow twitch") fibers and type II ("fast twitch") fibers within a given muscle, as well as presence and physiological predisposition to activate satellite cells
Whew. There's a reason it's called exercise science.
Very generally speaking, you can assume that larger muscle groups, particularly those with a larger cross-sectional area, such as the glutes, will be easier to get bigger, faster. It's also a fairly safe assumption that muscles that happen to have a higher percentage of fast-twitch fibers, including the pecs, lats, triceps, biceps, and quads, will also experience faster, more pronounced growth.
But as with all things fitness, the level of variability between individuals is a major confounding factor that could, quite possibly, explain why your calves look like scrawny chicken legs, while your best friend has no problem building gastrocs the size of bowling balls.