I'm curious how these breeds (e.g. bantam RI, bantam rocks, etc.) are developed. Is the standard-sized breed crossed with a bantam breed, or does the breeder select within a breed for smaller and smaller individuals? Or maybe both, or neither? Thanks!

Basically, the breeder selects a bantam chicken with similar characteristics and cross-breeds with the large fowl. He then selects for the traits of the large fowl and the size of the bantam. There exist natural bantam chickens that posess a gene for dwarfism, which can be passed to any large breed of chicken through cross breeding and selection.