It is possible to be thin and have big natural tits, you know. Think Aria Giovanni, think Veronica whatshername, think of my girlfriend.
Just at is possible to be thin in every other part but have a big ass, it is just as possible to be a twig with big tits. It's just not very common.
I don't really care about Nikki's tits either, but my guess would be real, because they looked rather soft to me in SOME pictures.
And, fucktard: Nikki is chubby? Where? She looks very tight and fit to me.