Well I must admit I already started discuss it with one of my friends and he suggest that perhaps more important than how long somepony is an alicorn is this in what age she(or he - who said that stallion couldn't become an alicorn?) become an alicorn.

And the fact, that Sunset become(or will become) alicorn after Twilight is 100% sure. they both in similar age, but it's possible that Sunset is a little bit older(she could become Celestia students before Twilight, what is very likely, even if not confirmed). So let's assume that when Sunset become alicorn she was 5 years older than Twilight when it happened(well Sunset could be about 2 years older than Twilight, and my calculations says that between "Magical mystery cure" and "Friendship games" passed about 3 years). the question is - "It is enough to have such difference in size?". And to answer it we should know what's the age difference between Celestia and Luna. The only think we can be sure is that it couldn't be more than about 20 years, but nothing more... also we don't know in what ages each of them become alicorn, but well this propably was young age(what doesn't fit this theory, but this assumption come from additional source).

But theory from "Past sins"(If You didn't read it i really recomend it - it's one of the best things I ever read!) could also explain it. This theory says that maturity, and also size of alicorn depends on quantity of magic in her(/his) body. Yeah, i know, this implicate that Sunset has more magic power than Twilight but... why not? The only problem is that episode "Twilight's Kingdom" denies this theory(if it would be true Celestia, Luna and Cadance should become a fillies after transfering their magic into Twilight, and she should become at least as big as Celestia).

Anyway it turned out, that this size difference could be explained(in one way or another).



And, what i forgot write aerlier, I think, that it would be better fit Twilight if her boots and crown were lighter(more silver, now they look more like steel).



And on the other hoof I think that if such thing (repleacin Celestia and Luna) happened it will be rather lont time after the show(like i don't know some hundrets years? Celestia and Luna doesn't seem to die quickly...), but is very likely that some day it really happened.