The kids actually grew a lot over the course of the series. The artist was pretty clever to show the time passing visually like that.You know, setting aside everything else, this is the chapter where I realized...Miyako is really short, isn't she? She's like maybe a head taller than Hana.
Also for all the people wondering if Mya-nee will be fine after college, the thing to remember is she's not a hikkikomori, she has social anxiety (but not even that severe social anxiety) and is generally antisocial, but she's perfectly capable of existing in society.