And look at the difference between Luna in Season 1: after the Elements of Harmony, then at the Luna in Season 2. Size difference, age difference.
That's what happens when ponies age. There is a gap between episodes, not a week gap, but a month or so. Luna had like 2 years to grow(the will be debated I'm sure)
I think each season is like the start of a new year, hence why we had "Winter Wrap Up" in Season 1, and before Spring in Season 2, we had "Hearts Warming Eve", both coinciding with the pony winter. In Luna's case, she just got back from the moon after being there in stasis for 1000 years as Nightmare Moon, so when the Elements were used on her, she reverted back to how she was 1000 years ago. For all we know, her age could be catching up to her quickly again, like that film "Jack" with Robin Williams in it. She's back to how she was a millennium ago, and she's always been the youngest sister to Celestia.
Plus new Luna is best Luna. Royal Canterlot Voice is awesome. Speaking of which, I should really show you guys a fic that explains A LOT. And I think it does it realistically. IT ALL MAKES SENSE after you read this fic. It explains: Why Discord is in that pose in the garden before he's released Why Luna REALLY talks in Shakespearian dialect and where the "ROYAL CANTERLOT VOICE" really came from Why Nightmare Moon really hated Celestia How a lot of the Mane 6's ancestors are involved in stuff that affected the future That Celestia really isn't a Troll Mastermind, it just turned out looking that way. Why Discord didn't take the Mane 6 seriously What exactly Discord did to the Mane 6 when they turned gray And a lot more. Not to mention it manages to fit in a great story and put everything in flashback format. This is one great fic, folks. And by one of my favorite authors on fanfiction.net. http://www.fanfiction.net/s/8008655/1/
