Spells can be finnicky and strange if not studied properly, and can lead to strange or huge results if not careful. Such is how Twilight Sparkle ended up in her current situation as she would try to cast a spell she discovered recently that she felt would help her better learn how to care for Spike. Turns out that it was a draconic transformation spell, and one surreal experience later she's left as a quite huge adult dragoness as she is now. And unfortunately for her, turns out the spell makes it so she's going to be stuck like this for a VERY long time before she can return to normal. So for now, she's stuck as a dragoness.

Safe to say, she's got a LOT to get used to, as she spends her time now reading books to see if she can figure out what all to do as a dragoness whilst also tending to her other normal duties and various other things of note. Because of course she would turn to books for literally every problem ever.