The column matrix is composed of cross-linked polymers with pores of selected sizes. Smaller molecules can enter the pores in the polymer beads from which larger molecules are excluded. Small molecules therefore have a larger three-dimensional space in which to diffuse, making their path through the column longer. Larger molecules migrate faster because they pass directly through the column, unhindered by the bead pores. B+D are outside of the separation range of the beads and elute together in the initial void volume. C and F are too small to be resolved by the beads and elute together at the end.