I found this usage of じゃない. Perhaps the translation is too loose in meaning.
A: Is your girlfriend cute?
B: Why wouldn't she be?
Does "かわいいに決まってるじゃない" mean the same thing as "かわいいに決まってる," just with more emphasis?
Is the following reasoning therefore correct?
Or is there a different nuance by using the negative form?
Furthermore, I suppose inferring the speaker's intentions come from context and the lilt in the speaker's voice, as in the following:
大きいね。Wow, it's big.
大きいじゃない。 It's big, isn't it.
大きいじゃん。 Damn, it's big.
Though without context, " 大きいじゃない。" simply means, "It's not big."
In English, if my translation is correct, the sentence,
It's big, isn't it?
should have a question mark. However, in Japanese, is it not required?