As far i remember, in anime the size of the eyes tend to show the AGE of the character, not the GENRE.
If a character is very young, he/she has very big eyes; (Ex.: Aruruu)
If a character is young yet mature, he/she has almost normal sized eyes. (Ex.: Hakuoro, Benawi)
If a character is old, he/she has small eyes (meaning he/she saw many things in his/her long life); (Ex.: Tsukuruu)
The twins eyes are bigger than Oboro, so that means they are younger than him.
Of course, there is exceptions, but i don't think that's the case now.
