There's enough space to account for some growth and shrinkage. We're talking about small but measureable amounts in actual size - the square cube law applies. Physiologically, the brain produces CSF continuously and the excess is reabsorbed - the fluid can buffer the changes in dimension within reasonable limits.
The study itself is interesting - it's now most definite that neural regeneration and growth occurs to a greater degree than previously thought. The ability of the human body to regenerate is far greater than what we originally believed. The sample size is somewhat small, but there are enough consistencies to be able to draw initial conclusions.
Unfortunately for something like this, there's no way a double blind study could ever be done. 😛
