Scaling relations between physical properties of the molecular clouds in NGC 628: (a) virial parameter as a function of CO peak brightness, (b) CO peak brightness as a function of cloud size, (c) linewidth as a function of size, and (d) luminous mass as a function of size. In all panels the headlight cloud is the red point, and the nine next most luminous (M_lum >  8.6 × 10⁶ M_⊙) molecular clouds are blue. The horizontal dashed lines in the top left panel represent virial parameters of α = 1 and α = 2, values that are indicative of virialized and self-gravitating clouds, respectively. The dashed line in the bottom left panel represents the size–linewidth relation reported by Solomon et al. (1987) for inner Milky Way clouds. The dashed line in the bottom right panel represents a mass surface density of Σ_cld = 200 M_⊙ pc⁻².