Anyway, while playing it, I observer their distinctive outline style, and realised it's not done by the game engine, and it can be easily done in Cinema 4D.
So, I made a test scene with Cinema and Unity game engine. It can be viewed here (requires Unity Web Player. Install it!)
I wrote a blog about the technique to share. It is quite lengthy, thus I wrote it in a separate page:HERE!
Well, it's been fun doing it. I hope you enjoy it too :)
As suggested by my friend YuanQin, a Unity user, here's a direct comparison between the Polygon Outline method vs. Unity Default Toon shading. As we can see from the images below, Unity Toon Shader gives unexpected result and many artifacts in some areas, compared to the Polygon Method.