For four and a half years, Equestrian magic experts tried and failed to find a cure for the “Big Butt” spell that had affected every mare in Equestria. However, when the culprit behind the incident was finally caught, and they relinquished their findings to the royal authorities in exchange for a lighter sentence, it at last gave magical researchers a hoof in the door. With the original spell in her possession, Princess Celestia was confidant that creating a way to counter it would be a piece of cake.
 
The task was delegated to a team of royal researchers based at Celestia’s School for Gifted Unicorns. Included on the team was Twilight Sparkle, an extremely powerful unicorn who was considered by many to be the Princess’s most faithful student, and Moon Dancer, a devoted young researcher who had always shown great potential in her magical studies.
 
Ever eager to prove her dedication to the Princess, Twilight Sparkle offered up her over-sized flank as the guinea pig for the experiments. After a few failed attempts, the counter spell took effect, reducing Twilight’s rump to a smallness that hadn’t been seen in any mare for almost five years. However, not wanting to be known as the mare with the smallest backside in Equestria, Twilight requested to have her big butt restored until the counter spell could be cast on a worldwide scale. As the incident report shows, Moon Dancer’s execution of the spell was slightly off, and Twilight Sparkle ended up receiving much more backside than she bargained for.
 
Twilight’s butt was quickly reduced to a more sane size with no harm done, but those who have been lucky enough to see her purple plot in person often make the claim that it’s still slightly - just slightly - bigger than the average mare’s. As of yet, these rumors are unconfirmed, and Twilight Sparkle herself hasn’t made any public effort to dispel the speculation - if anything, she’s been encouraging it. But if there’s one thing we know for sure, it’s that Twilight certainly does not have the smallest backside in Equestria. Not by a long shot.
 
(For context)