@nightshroud96  
In my view, all alicorns grow to Celestia’s size, which is why Twilight was that size in the last episode. Luna should have been Celestia’s size if not for her transformation into Nightmare moon which trapped her in that body and prevented her from fully developing into a full-grown alicorn and when she was reborn as Luna again after the defeat of Nightmare Moon, her power level it was too low, so its mane didn’t move and its color was lighter. In the last episode, Luna and Celestia were not shown, but probably Luna would already be fully grown due to the time that passed and the same happened with Cadance who is also fully grown despite not appearing in the last episode.

@Phantom Rider  
Luna fans will nitpick anything it seems to prevent others from enjoying the other Princesses

@Phantom Rider  
The problem is Twilight.  
She is apparently all Celestia-size after the timeskip and pretty much it raised alot of questions on Alicorn biology.  
And thus it feels like Luna got the short end of the stick due to that.(Yes, Luna has her flowing mane now but pretty sure that doesn’t mean she is “fully grown”, just her magic is growing. I mean, Twilight’s flowed when she got the princesses’ magic from Celestia’s moronic plan in Twilight’s Kingdom and then briefly flowing just before she friendship-nuked Sombra in season 9)
 
Like really.. Twilight’s case really brought up some confusion/questions about Alicorns there.

@Phantom Rider
 
The fact that a relatively short time led to Twilight growing to Celestia’s size.

@Eurvos Morgenbrise  
She’s still big with floating hair. What is so wrong with her being smaller than her sister?

@Eurvos Morgenbrise  
Which I really hate how they didn’t bother showing us how are the sisters doing post-timeskip.  
Their retirement bullcrap is literally what caused the future to be like it is now.  
Like a huge part of the season there.

@Background Pony #2562
 
Because her “ultimate” appearance was never canonized or given a canon counterpart.

How does Luna get the “short end” when she is worshipped by almost everyone in the fandom? Seems like Luna fans will literally do anything at this point to prevent anyone from appreciating the other Princesses.

@nightshroud96
 
Or maybe he left in the 2 months that have passed?  
Given another user I know is definitely gone and has a similar label on her username, it’s safe to say he’s definitely gone.
 
Also we didn’t see Luna post-timeskip and Lauren did say she wasn’t fully grown as a result of Nightmare Moon.

@Eurvos Morgenbrise  
Feels like Luna got the short end of the stick where apparently she’s so shorter than Celestia and now Twilight too.
 
 
Pretty sure the guy is still active. Its just how his username was now for some reason.  
I checked and his last comment was about 2 months ago.  
If he was gone, then I wouldn’t be able to look at his account if he deactivated it.

@nightshroud96
 
Especially since it only took “many” months for Twilight to do it as seen in The Last Problem.
 
Also you know that guy’s gone, right?

@deactivated843b8160732  
Really wished the show could have touched on that.  
Like having Luna finally grow to Celestia-size.

@Phantom Rider  
Well you have to remember, Lauren said that turning into Nightmare Moon froze Luna’s growth.
 
Celestia on the other hand likely finished hers before that time, considering she looks the same in the “Princess Twilight Sparkle” flashbacks as she does in the Present Day. (1)
 
“Ultimate Luna” (2) is basically what folks imagine Luna to look like once her growth completes.
 

 
(1) Granted, that might just be the show creators re-using Celestia’s model to save time rather than drawing any “young Celestia” models that might never be used again. However, it does line up with the comics as “Reflections” also shows Celestia looking the same 1000 years ago as she does in the Present.
 
(2) I personally call it “Moonlight Dream” as a contrast to Nightmare Moon.

@Mugen Kagemaru  
I always thought it was for more ultimate-er redesigns that usually combine all her forms for maximum goddessliness. Just taller isn’t enough.

“ultimate luna” tag for Celestia-size Luna.
 
Also seems to be an edit of >>479531