Reshiram and Zekrom are bigger than kyurem and it would have to be a new type with dragon, fire and electric moves the eyes would have to be purple.
Yeah... Because splitting from Kyurem would somehow make Kyurem larger right..? Also, their eye colors reference each other's secondary type, Reshiram's eyes are blue, referencing Kyurem's ice, Zekrom's eyes are red, referencing Reshiram's fire, Kyurem's eyes are yellow, referencing Zekrom's electricity, as for having to be a "combined" type, Regigigas made the other three Regis, and he is normal typed so...
Like I said, no one knows yet what Kyurem is, though after Platinum, most think it will be like Giratina and, somehow, be connected to them both, and probably be the next version mascot.
Maybe they split from Kyurem, maybe Kyurem is just what is left after the split, maybe he isn't involved at all... Until Pokemon's patented tie-in/make a lot more money game comes out, then we'll probably get more answers, until then, it's all 100% speculation, though I tend to believe that they came from Kyurem myself...