It is some of both. Mia's not particularly tall — at 156cm she's the same height as Yu or Yoshiko — but she is a year younger than Rina; meanwhile, Rina is the shortest girl in the franchise at 149cm, a full five centimeters shorter than Nico.
(The next ones up from Rina are Rurino at 151cm, Hanamaru and Natsumi at 152cm, and Leah at 153cm; Tsubasa, Ruby, and Setsuna are all the same height as Nico.)
To put her height in further perspective, if Rina were over in Idolmaster Cinderella Girls, she'd qualify for the spinoff that's all elementary school girls.