A view at another world...or Celly seeing her future?

Its a mirror that shows what you truly desire

"Isn't it about time you dispelled that enchantment my dear? Let the world see our true self. What could they do, force us onto a diet? I think we both know it's FAR too late for that to work."

Heh, I can so see that happen.

My personal headcanon is that Celestia fell into a deep depression after banishing Luna and went on a binge of cakes, pastries and other sweets that lasted for years. When she finally pulled herself together she was too big to ever loose the weight conventionally, so she came up with a spell to compress/hide all the fat away in an extra-dimensional 'pocket'. She appears normal with her thin profile but still weighs the vast tonnage that has only grown over the centuries due to her love of cakes and other sweets. (Like, she's reaching the density where her hooves are starting to crack the stones she walks on.)

She knows that one day the spell might fail on it's own as she's had to reinforce it several times, but she's yet to work up the courage to show her real self to the world. Partially out of worry what other might say, partially out of swamping the entire palace in an avalanche of her own blubber.

And I have a feeling that her subjects will probably accept her, leading to rampant obesity, and a lot of deep fat voices.

It will take some time, but eventually. And though Luna might laugh initially I imagine she gets annoyed as Celestia's popularity only seems to grow along with her weight. So she resolves to no longer be the little sister and outshine Celestia for once in sheer size. Keeping it secret at first until she reveals herself at the next Gala to have grown to a small hill of alicorn blubber. ^_^

At that point, they might as well hold the Gala on both of them, to avoid anyone getting pinned under their blubber. ^ _ ^

Also, with a certain Draconequus enjoying the show...and sporting tonnage himself~

Right, two years late to the party, but this is such a great premise

Oh C'mon. That is how everyone feels when they see themselves in a mirror.

"Sister, how do you stay so thin with all you eat?"
"Oh, I store my extra weight in the Mirror Dimension."