They did record significant shrinkage in the users, in fact a larger decrease in size among users than the increase in size among non-users.

Here's the bit: ...This effect was significant in the left thalamus, at a volumeric increase of 26.66 mm3 in nonusers and a decrease of 181.39 mm3 in users, and highly significant in the right thalamus, at an increase of 32.84 mm3 and a decrease of 264.48 mm3 in nonusers and users, respectively.