Why Luna is smaller than Celestia ?

[The Kaeya Simp 13,969]

Maybe one of Luna and Celestia's parents was tall and one was shorter and Luna might have just got more of the genes from the parent that was shorter than Celestia

[Thrond 3,257]

After the moon thing, I guess the elements reduced her to some sort of younger state because... it’s more innocent? I dunno. 

Otherwise, she’s probably just smaller because of genetics. Stuff like that happens.

Edited June 1, 2018 by AlexanderThrond

[Mortar 143]

I haven't read the responses, so it's probably been said before.  She Celestia's younger sister.  How much younger we don't know, so it's reasonable she'd be smaller.

[Tenebrae 14]

Simple. She is just shorter than her sister. Celestia seems to be the one who is abnormally tall. Besides pony heights vary from individual to individual. Big Mac, Starswirl, Fleur are all tall ponies and Rock Hoof is enormous.

[Im_not_crazy_im_just 1,814]

This is what would happen if she was bigger than celestia... That's why.

[Lord Valtasar 12,685]

it could work like humans, not everyone has the same height, its not only depended on age,
as for her becoming young during her reformation, maybe the proccess of cleansing nightmare moon was particularly draining, so she had to become like that to preserve energy, once she was healed, she turned back to normal size quickly enough,she didn't have to wait a thousand years again

[Denim&Venöm 16,529]

The moon is smaller than the sun.

That's why. 

[Senko 459]

I think she's just the smaller of the siblings like Twilight is smaller than Shining Armour. I did however read one fun fanfic where both Luna and Celestia are the size of normal ponies they just use magic to alter their appearance to a more appropriate one for their roles as princesses which is also why they have those floaty, glowy manes. I think its Scootaloo who looks in on a sleeping Luna and see's her in that teenage form of S1 and remembers Sweetie Bell saying Rarity also looks very different without her make up on.

I know it doesn't work with the images of a sleeping Luna but I rather like the theory.

[RDDash 18,867]

Maybe she is just physically smaller, and in reality Luna is about in similar age group as Celestia.