collapsed cloud is independent of radius and proportional to the surface density of the disk, with the normalisation set by the collapse factor and under the assumption $Q$=1. @Hopkins12b show that this model provides an reasonable fit to giant molecular clouds in the Milky-Way, and further, suggests that the surface density (and hence surface brightness) of clouds should increase with the surface density of the disk.

Using our estimates of the stellar and gas masses and spatial extent of the galaxies in our sample, we derive disk surface densities of $\Sigma_{\rm
  disk}$=1.1$\pm$0.4$\times$10$^{9}$M$_{\odot}$kpc$^{-2}$, and hence expect the mass surface densities of the star-forming regions that form to have mass surface densities of $\Sigma_{\rm clump}\sim
$10$^{10}$M$_{\odot}$kpc$^{-2}$. It is instructive to compare this to the average mass surface density of the clumps. For example, assuming that their velocity dispersions are virial and adopting $M_{\rm clump}$=$C\sigma^2 r_h$/$G$, using the average velocity dispersion and size of the clumps (Table 2), we derive an average clump mass surface density of $\Sigma_{\rm
  clump}$=8$\pm$2$\times$10$^9$M$_{\odot}$kpc$^{-2}$ with $C$=5 (appropriate for a uniform density sphere). Although this calculation should be considered crude as it is unclear whether the velocity dispersions we measure are virial, it is encouraging that the predicted surface 