I have talked to some hatchery people about those trout and I was told they grow big because they are sterile. The feed them the same as a regular trout but they continue to grow all year long without any stress from spawning. They also said they make them sterile by raising the water temp when they are eggs. Acording to them this also happens in nature when it gets warm durring the trout spawn.
Kinda like when you cut the balls off a bull and make a stear. They get big and fat faster.