  A key problem in protoplanetary disc evolution is understanding the
efficiency of dust radial drift. This process makes the observed dust disc
sizes shrink on relatively short timescales, implying that discs started much
larger than what we see now. In this paper we use an independent constraint,
the gas radius (as probed by CO rotational emission), to test disc evolution
models. In particular, we consider the ratio between the dust and gas radius,
$R_{\rm CO}/R_{\rm dust}$. We model the time evolution of protoplanetary discs
under the influence of viscous evolution, grain growth, and radial drift. Then,
using the radiative transfer code RADMC with approximate chemistry, we compute
the dust and gas radii of the models and investigate how $R_{\rm CO}/R_{\rm
dust}$ evolves. Our main finding is that, for a broad range of values of disc
mass, initial radius, and viscosity, $R_{\rm CO}/R_{\rm dust}$ becomes large
(>5) after only a short time (<1 Myr) due to radial drift. This is at odds with
measurements in young star forming regions such as Lupus, which find much
smaller values, implying that dust radial drift is too efficient in these
models. Substructures, commonly invoked to stop radial drift in large, bright
discs, must then be present, although currently unresolved, in most discs.
