An average birth weight for kittens is about 3 ½ ounces, depending on breed and litter size. During the first weeks of life, a kitten’s body weight may double or even triple. Gaining ¼ to half an ounce daily until they are weaned is considered healthy.n average birth weight for kittens is about 3 ½ ounces, depending on breed and litter size. During the first weeks of life, a kitten’s body weight may double or even triple. Gaining ¼ to half an ounce daily until they are weaned is considered healthy.