I honestly think they will be pretty close. Gotta keep in mind practicality: filling a world of significantly bigger time with meaningful content takes more resources. So there is a point where genre expectations and the demands of development start to limit you. Now both games compensate with faster movement speeds, one with a car, the other with flying mecha. So I think both will be closer in size than people think.
Also, I would like to point out Xenoblade's story may have a lot of roots in tropes, but those tropes are very genre specific so I kinda missed them myself (haven't played many jrpgs prior) xD . And I felt that the uniqueness of execution was very cool. It's kinda like Renaissance artists: most were telling the same or similar stories with their art (in fact, there were like four or five major themes grand total for most of it xD ), just a question of who can tell it better and more uniquely.
