Battery size different?

I suppose that we'll know tomorrow, but I haven't seen any discussion as to whether the 42mm version will have a larger battery than the 38mm. Since the chip is the same in both it would make sense that the larger case would accommodate a larger battery (that would last longer). I'm thinking of my iPhone 6+ and the larger battery that it has over the 6 .... It would just seem that they would fill the 'extra space' with something useful.