Context : 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 .

Aims : 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 .

Methods : 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 .

Results : 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 } \mathrm { 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 } < 1 \times the characteristic radius and a peak SNR > 10 on the ^ { 12 } CO 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 ^ { 12 } CO flux will contain most of the disk mass .

Conclusions : 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 .