A bantam is any small or miniature fowl; the term bantam refers most commonly to small breeds of chickens. Many standard chicken breeds have a bantam counterpart, sometimes referred to as a miniature. Miniatures are usually one-fifth to one-quarter the size of the standard breed, but they are expected to exhibit all of the standard breed's characteristics. Some examples of breeds with bantam counterparts are the Plymouth Rock, the Australorp, and the Wyandotte.
A true bantam has no standard-breed counterpart. Examples of true bantams include the Japanese Bantam, Dutch bantam, the Belgian bantam, the Rosecomb, and the Sebright.
Bantams have become increasingly popular as pets as well as for show purposes because they are smaller and have more varied and exotic colors and feather patterns than other chickens. They are great for smaller backyards as they do not need as much space as other breeds. It has been said that Bantam hens are calmer than standard hens. Bantam hens are also used as laying hens, although it takes two to three Bantam eggs to make one standard chicken egg.
First, the terminology. While the large chickens are often called standards, the correct term is largefowl. Remember, most of the bantams actually are in the standard. The term standards was coined back when bantams were very uncommon, were looked down upon, and were not yet recognized or standardized. It helped differentiate the standards from these new miniature versions of the same breeds.
The idea that bantams have "more varied and exotic colors and feather patterns than other chickens" doesn't make much sense, because almost all largefowl have a bantam counterpart. It is true that there are sometimes a few more recogized varieties in bantams than in largefowl, but the breeds which do recognize more bantam varieties, recognize them in colors which are to be found in some other breed of largefowl, somewhere. There is nothing more varied or exotic about them. Even the true bantams are found in varieties which can be all be found in some breeds of largefowl. Nothing new there either.
The oldest ASOP I own is an 1875 edition in which are listed several bantam breeds/varieties. So it seems they've been standardized/recognized for quite some time.
The 1947 Standard does not use the phrase "Large Chickens", the 1958 does so in there somewhere that phrase first appeared [at least in official APA terminology].