If they are bulkier than your other chicks at the same age, they could be Cornishx meat birds.
Find out what hatchery supplies your feed store, and look at that site to see who resembles your chicks. Feed store staff may not have any idea what breeds look like, and often the chicks in the bins are mixed up too.
It's best to know what chicks of the breeds you buy actually look like before taking them home.
Mary
Maybe white Plymouth Rocks?