The storage of a mipmap is around 4/3 of the original size. Take a 256 * 256 map for example, the lower level mipmaps are from 128 * 128 to 1 * 1. The ratio of the area between levels is 1/4. So the amount of the mipmap is (1 + 1/4 + 1/16 + ... + 1/2^n) = 4/3 (here 2^n is the original size).

The storage of a mipmap is around 4/3 of the original size. Take a 256 * 256 map for example, the lower level mipmaps are from 128 * 128 to 1 * 1. The ratio of the area between levels is 1/4. So the amount of the mipmap is (1 + 1/4 + 1/16 + ... + 1/2^n) = 4/3 (here 2^n is the original size).