i thought chibi was suppose to be "little people with big head"?? how is this chibi?

Chibi just means small - remember Sailor Chibi (mini) Moon

There seems to be a fan-meme where Natsuki and Shizuru have kids which are chibi versions of them (rather creepy if you ask me, but...). See post #77778, post #60588, etc.
Judging by this picture, the Natsuki lookalike is called Shizunatsu, and little Shizuru would be... Natsushizu?