“Most spells would, but this one is different.  It was meant to transport a unicorn to a different planet, and bring her back if she was unable to cast a return spell on her own.  Starswirl was always interested in other worlds, and since he explored some of them himself, he wanted to assure his safe return.  He probably didn’t consider what might happen if a unicorn found herself unexpectedly returning with four times the mass she’d started with.  Once he got more involved with portals, he probably stopped working on the spell.  Much of his research has been lost, unfortunately.  This spell was among Clopurnicus’ records, or else we wouldn’t know of it.”