@buttbadger  
I like to think Luna was able to revert back into a ‘super alicorn’ state on her own, though there’s nothing canon to prove it either way yet.
 
Here’s a little chart about your theory:  

@Amethyst_Crystal  
It’s because there’s two alicorn states: the normal state (Luna s1, Twilight and Cadance) and the “super” alicorn state (Luna s2+ and Celestia)  
“Super” alicorns have those magical wavy and sparkly hair, are way much taller than regular ponies and alicorns, have stronger magic and are immortals. And only regular alicorns can potentially become super alicorns.  
When Luna got cursed by Nightmare Moon spirit and when this spirit flee away, it made Luna go back to her alicorn state, for some reason. And i guess Celestia helped her become a super alicorn again when she apologized. And when you think of it, Celestia also has a regular alicorn state where she has pink hair. And i think she’s also smaller in that state.
 
Well, this is my own theory, i might be wrong :p

Who doesn’t want three wings?

…  

 
This is one of the better shots in the show comparing the relative size of the alicorns.  
S2+ Luna seems a bit bigger than S1 Luna, but…  
They were sometimes a bit inconsistent with size scale back then, too.

Holy shit, she’s TINY

So both sisters are terrible shots?
 
That was a very cute moment just between the two of them, too bad we haven’t seen more of these.

Dat face Luna does at the end :3

1000 years of malnutrition does that to alicorns …

@JP  
Luna hasn’t actually changed size - the show staff even said as much at one point. It’s just that we don’t get that many shots in the series that give a good, direct comparison of their heights.
 
 

Blessed by the Swanlestia.

Luna was so tiny back then. And looked so adorable.

Aw, that’s so sweet.