Body type has as much to do with metabolism as the actual shape, by my understanding. Ectomorphs have a very hard time putting on weight at all. I struggled, struggled, struggled to add any sort of meaningful weight in high school. Tried my ass off, for football, too. Now my metabolism has slowed enough that I can control my weight directly (adding muscle if I want or slimming down if I want) simply by diet and working out. To be perfectly honest, that just wasn't the case for me before. Couldn't gain no matter what I did.