To identify the structural role of alloying element M (M = Ti, Ga, Co, Fe) on the glass-forming ability (GFA) of Cu50Zr50 base alloy, the atomic structures of the binary and ternary metallic glasses were examined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS curve-fitting analysis indicates that the main structural difference among the metallic glasses is in the atomic packing density of Cu-centered clusters. The relative shortening of the Cu-M distance is closely related to the heat of mixing between Cu and M: the more negative the heat of mixing, the larger is the shortening of the Cu-M distance. Based on a systematic analysis of the component properties and GFA data for Cu-Zr based alloys, it is suggested that alloying elements that bring a more uniform distribution of atomic size and possess strong chemical interactions with the main components should be selected in developing large-size bulk metallic glasses.