For those who are new on Bandai SD series:
SD Gundam (Japanese: SDガンダム Hepburn: Esu Dhī Gandamu?, short for Super Deformed Gundam) is a media franchise that spawned from the Gundam franchise. SD Gundam takes the mecha (and characters) from Gundam and expresses them in super deformed and anthropomorphic style
source: https://en.wikipedia.org/wiki/SD_Gundam
basically,SD are the big head and a small body character/figure
(sorry for blurry image, i took most of the figure box with my phone camera)