IGN says Yoshio Sakamoto (Metroid, WarioWare, Tomodachi Life, etc.) and his team were behind Miitomo's development. Nintendo declines to talk about Sakamoto's team size or how long Miitomo spent in development.
This pretty much further busts the Tanabe/Retro Studios fallout rumor. Sakamoto couldn't be at Retro Studios since he was busy with Miitomo's development.
EDIT - Miitomo has passed one million downloads on iOS and Android combined in Japan in just three days:
Edited by Ngamer01, 20 March 2016 - 07:32 AM.