All the characters in Mirai Nikki follow more or less the same style, with one exception - Eighth, who has a strangely oversized head.
Spot the culprit
Why is this? Is there a specific reason Eighth has this appearance?
I wasn't able to find anything that explicitly stated as to why her character design is that way. It is apparent that a lot of the fans do not understand why her design is that way and do not like the way she looks.
I will try to explain my interpretation as to why she might have been created to look that way.
During the series, a good number of the diary owners are sociopaths that enjoy killing other people. There are a few exceptions to the trend like Yuki, Keigo, etc. Kamado is also part of this group not wanting to have anything to do with the game, but her orphan children want her to win and form a little army to protect her. The character design that was chosen shows that she doesn't look threatening in any way and leads most people to believe she is harmless.
As we all know, Kamado runs an orphanage that caters to quite a few children. This design could imitate how some children perceive their mother. A mother or father is a very big presence in a child's life, and this over inflated head design can reflect how a mothers head may look to a small child when she is bending down talking to a child or infant.
Taking those factors into account, that could lead to an exaggerated character design. Note that this is in no way 100% fact, as it is my general interpretation as to why the artist may do this.