Bit depth describes the file size of an image by orders of magnitude.

When wrangling with file size versus image quality, it’s often important to minimize the bit depth of an image while maximizing the number of colors. To calculate the maximum number of colors for an image of a particular bit depth, remember that the number of colors is equal to two to the power of what the bit depth is. For example, a GIF can support up to eight bits per pixel, and therefore can have as a many as 256 colors, since two to the power of eight equals 256.