By those point Mielle's already like over 15. It makes sense for her to have developed. I think the artists have been maturing her ever since Aria's growth spurt so they can stand next to each other and not look ridiculous. I mean, in the past 20 chapters or so she didn't look small to me.
@AlessaO I'm kind of late, but I believe it has to be a 'letter.' She did say, 'wait, does this count as a letter?' when she first sent her second message. So...perhaps, but probably not.