There are only three ways brain tissues will "swell," get physically larger. Either a) the cells themselves have expanded, which because of the plasma membrane is very difficult, b) fluids have concentrated in the tissues, or c) the cells of the tissue have replicated.
For brain tissues, all three of these events are pathogenic. It is naively presumptuous to suggest that because the drug caused any brain tissue to "expand" some propitious or therapeutic outcome would result. It would be either some sort of edema, dysregulation of the cytosol, or a tumor.
A mechanism of operation (MOA) offered in utter desperation. Really swell stuff.