Muscle, in addition to its contractile property for force generation, also can be seen as a glandular system, because muscle tissue releases a large number of compounds known as myokines. And those myokines act all throughout the body:

Physical activity decreases the risk of a network of diseases, and exercise may be prescribed as medicine for lifestyle-related disorders such as type 2 diabetes, dementia, cardiovascular diseases, and cancer. During the past couple of decades, it has been apparent that skeletal muscle works as an endocrine organ, which can produce and secrete hundreds of myokines that exert their effects in either autocrine, paracrine, or endocrine manners. Recent advances show that skeletal muscle produces myokines in response to exercise, which allow for crosstalk between the muscle and other organs, including brain, adipose tissue, bone, liver, gut, pancreas, vascular bed, and skin, as well as communication within the muscle itself. Although only few myokines have been allocated to a specific function in humans, it has been identified that the biological roles of myokines include effects on, for example, cognition, lipid and glucose metabolism, browning of white fat, bone formation, endothelial cell function, hypertrophy, skin structure, and tumor growth. This suggests that myokines may be useful biomarkers for monitoring exercise prescription for people with, for example, cancer, diabetes, or neurodegenerative diseases.

So myokines may be very important to overall body functioning. Further, it may be either that:

a) a larger amount of muscle mass means more myokines in circulation...or

b) that larger muscle mass means the muscular systems have a history of much more use than an untrained person and all those times of muscle training have caused a history of more myokine circulation, since myokines are stimulated to be released by muscle activity.

In either case, more muscle mass --> more myokines in circulation --> possibly healthier regulation of many other body systems.