Board Thread:Naruto Discussions/@comment-5054236-20150717003436/@comment-5273810-20150717005442

It is unknown. My guess is perhaps like Obito explained earlier on how the Senju got the body of the Sage of 6 Paths while the Uchiha got the eyes of So6P is more literal than we realized.

The Uchiha would have gotten a downgraded version of Sage's eyes(the Sharingan) and the Senju got a weaker version of the Sage's body. So when Indra and Asura mix their chakra together each one gets they get the equivalent of So6P's full power in that area so in Indra's case he gets Rinnegan while in Asura's he gets increased power in his body and chakra reserves.

Again just conjecture but it does have some basis on known facts.