However, the large size of the epulos was still unexpected. The small size of most bacteria is owed to their limited abilities. Since they have few ways to transport nutrients across their cell membranes, they rely on diffusion to move food into their cells, and wastes out of them. This process of diffusion is limited by the surface area of a cell, which is the space that a cell's surface would occupy if it were stretched out flat. As a cell gets bigger, both its volume and surface area increase, but its surface area increases more slowly than its volume. Above a certain size, there is not enough surface area to absorb all of the nutrients that the increasing cell volume needs. So, the limit of a bacterium's size is related to the proportion of its surface area to its volume.