What color are the earlobes?
If they are silvery blue or white, then it's a white Leghorn. Some Leghorn strains have smaller combs, although mine have combs so large they flop down and cover the eye.
Definitely not a white Cornish--can you even buy a white Cornish anywhere? Not the right feather type for a Cornish, anyway. They are big-breasted birds with hard feathers.