Their eggs take 21 days to develop and hatch. I have bantam chickens such as silkies, d'uccles, and seramas who weigh anywhere from about 12 oz up to 3-4 pounds. Their eggs also take 21 days to develop and hatch. A chicken, large or small (bantam), will sit on eggs for 21 days to incubate and hatch them. If the eggs are NOT fertile, then of course they won't hatch, and sometimes the chicken will continue to sit well past the 21 day mark, hoping to hatch them. Here's a pic of a mix of bantam and standard chicken eggs.