I sold a pair of turkeys and the boy asked me "how do you get your turkey hens so big?" The only thing I could think to tell him was because we feed them good quality feed. I would say better than 90% of the people in my area (I am being generous with my estimate) feed straight scratch or corn. Nothing nutritious about that. Muscle is protein, and you can't build protein without protein. A bird can get fat, yes, on a grain diet, but it will not develop a good body composition with adequate muscle without adequate dietary sources of protein from day 1. Not to mention poor egg production. Unless a bird is 100% free-ranged, you sould not feed straight scratch or corn. If a bird is free-ranged, it will supplement what you feed with natural protein sources in the warm months. But should still be given a good quality feed in the winter.
Sorry for ranting.