I updated this calculation by adding curves for contribution through bulk loss angle and shear loss angle separately. Rana suspected that shear loss and bulk loss should behave differently with the change in beam radius on the mirrors. But apparently the Hong et al. calculations do not suggest that way. I checked this analytically too. The definitions of Eq. (96) of power spectral densities of coating thickness fluctuation of a particular layer due to Bulk or Shear loss angles have the same dependence on the effective area of the beam on the layer.

The only thing different between the final contribution from these different fluctuations is in the transfer functions mentioned in Table I from bulk and shear noise fields to layer thicknesses and surface height of coating-substrate interface. These are also plotted in a second curve to get an idea of these transfer functions.

Along with the effects of layer thickness and surface height changes to final mirror displacement (effective) via phase change of reflected light is given by parameters q^B and q^S as defined in Eq. (94). These are plotted in the third plot and show the real difference in contribution from bulk and shear. This is in stark contradiction with what Ian just told me. They believe that for a Gaussian pressure profile, the energy stored in shear strain is 3 times higher than that stored in bulk strain. For comparison with a figure of the paper (fig.7.), I plotted the square root of these transfer functions in the fourth plot. However, the paper plots these for Silica and Tantla, not AlGaAs.

My conclusion is that at least in Hong et al.'s treatment, the effect of beam area on the mirror is equal to both bulk and shear contributions (Eq. 96).