  The extent of the gas in protoplanetary disks is observed to be universally
larger than the extent of the dust. This is often attributed to radial drift
and grain growth of the mm grains, but line optical depth produces a similar
observational signature. We investigate in what parts of the disk structure
parameter space dust evolution and line optical depth are the dominant drivers
of the observed gas and dust size difference. Using the thermochemical model
DALI with dust evolution included we ran a grid of models aimed at reproducing
the observed gas and dust size dichotomy. The relation between R_dust and dust
evolution is non-monotonic and depends on the disk structure. R_gas is directly
related to the radius where the CO column density drops below 10^15 cm^-2 and
CO becomes photodissociated. R_gas is not affected by dust evolution but scales
with the total CO content of the disk. R_gas/R_dust > 4 is a clear sign for
dust evolution and radial drift in disks, but these cases are rare in current
observations. For disks with a smaller R_gas/R_dust, identifying dust evolution
from R_gas/R_dust requires modelling the disk structure including the total CO
content. To minimize the uncertainties due to observational factors requires
FWHM_beam < 1x the characteristic radius and a peak SNR > 10 on the 12CO
emission moment zero map. For the dust outer radius to enclose most of the disk
mass, it should be defined using a high fraction (90-95%) of the total flux.
For the gas, any radius enclosing > 60% of the 12CO flux will contain most of
the disk mass. To distinguish radial drift and grain growth from line optical
depth effects based on size ratios requires disks to be observed at high enough
angular resolution and the disk structure should to be modelled to account for
the total CO content of the disk.
