Q: What are bantam chickens?
A: Bantams chickens are miniature chickens. Regular-sized chickens are often called "standards," but in reality the correct term for the larger chickens is "large fowl."
Generally speaking, bantam versions of large fowl breeds tend to be 1/3 to 1/2 the size of the larger birds. Below you see two cochins, one a bantam and the other a large fowl cochin.
But even bantams vary in size. The tiniest seramas might weight just a few ounces, while larger bantams may weight more than two pounds. Put a Serama beside a Silkie, for example, and the Silkies look like large fowl!
"True bantams" are chickens which don't have a "large fowl" version, such as d'Uccles or Sebrights.