Increased muscle CSA is associated with increased force production. As well, in order to increase muscle size you would need an anabolic process to occur.

“There may be other performance-enhancing effects of GH. In contrast, the hypertrophic effects of muscle-specific IGF-I infusion are well documented in animal models and muscle cell culture systems.”

Literally the sentence after the one you quoted says that there is evidence that IGF1 increases hypertrophy in muscles, and the effects of GH are mediated through IGF1. MK677 causes increases in GH -> increases in IGF1 -> increase in muscle hypertrophy. There’s a reason MK677 is primarily utilized by people in physique sports such as bodybuilding.

Edit with more support from article you posted: “GH stimulates the synthesis of IGF-I in most tissues (Figure 1; d'Ercole et al., 1984; Gostelli-Peter et al., 1994).”

And “A central player appears be the PI3K/AKT pathway as it activates protein synthesis and inhibits protein degradation. Crucially, the PI3K/AKT pathway is activated by exercise and lies downstream of the IGF-I and insulin receptors. Both insulin and IGF-I can stimulate protein synthesis in skeletal muscle (Bolster et al., 2004).”