I know there are somepeople on this board thatknowalot more about this than myself but, This is how I undestand it -
IGF-1 bonds to certain receptors on cells and activates the AKT genes which allow for better protein synthesis and lead to tissue growth. Cells grow and multiply. IGF-1 also inhibits programmed cell death
Insulin - pretty much allows your body to process much more food/protein. It allows more blood glucose to be used by the cells for energy. It also allows more protein to enter the cell. If your body is taking in too much food your blood glucose levels rise and your body releases insulin, but only so much. Thats why when we "bulk" we dont eat 10k calories a day, most would be wasted. The more insulin, within reason, the more food our bodies can process.
IMO