As part of my head-canon universe [Which includes Everlight Everlasting], as time went on when Luna didn't use her magic for extreme purposes like raising the moon anymore along with her sister, her body changed and lessened in it's regal, powerful figure. Her body and hair changed to a younger facet since Luna was no longer needing to use an immersive amount of magic compared to Princess Twilight and Cadence.

A part of an Alicorn's anatomy depends on the magical output and input they present. Over time, if an Alicorn uses a large amount of magic over a long period of time, they're body will slowly change to present their level of magic usage. The main reason for Princess Celestia's larger form compared to Princess Luna was because of the mass amounts of magic she needed to output from raising and setting the sun and moon. Yet after Princess Luna proceeded her daily royal duties, her larger form stayed intact until much, much later into her retirement because of her 1000 years of daily sun and moon duties.
Overall, Celestia and Luna's height become compared to Princess Cadence since they still contain high amount of magical input to remain a higher then average stater.

Princess Twilight however out heights them all from her mass amounts of both magical output and input over a long period of time. The longer an Alicorn uses a high amount of output magic, the longer it'll take for their form to reduce in anatomy, appearance, and size.
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