Lightness and thinness
Bone china can be made thin without becoming fragile. That is why a fine teacup weighs so little.
Pick up a fine bone china cup after a stoneware mug and the difference is immediate. Bone china is not a lighter substance; there is simply less of it.
Thin because it is strong
A plate or cup has to be thick enough not to break in use. The stronger the material, the thinner it can be for the same working strength. Bone china’s high bending strength means designers can use a thinner wall than they could in earthenware or most stoneware. Thinner walls mean less material, and less material means less weight.
The relationship is not proportional. A plate’s ability to carry a bending load rises roughly with the square of its thickness — so modest gains in material strength allow useful savings in thickness. Strength explains this with an interactive model.
A design choice
Bone china does not have to be thin. Hospitality ranges are often made heavier, with thicker, rolled rims, because a busy restaurant asks different things of a plate than a tea table at home. Fine teaware is often made as thin as the maker dares. Both can be good bone china.