The depth of the PS1 galaxy catalogue is shallower than the stellar catalogue simply because galaxies, being extended sources, have lower surface brightnesses. In the figure below, copied from Fig 12 of Metcalfe et al (2013), we show how the depth
of simulated stars (green and blue) compares to the depth of simulated galaxies (magenta) on a stack of science verification data (for more details see the paper).