I...sort of get you. Yes, it would be bigger than the 12-inch MacBook, and even the 13-inch Air/Pro. But why is that a bad thing? I think that is sort of the point. Sure, it removes the portability and ease of use of the 10-inch iPad, but it will probably be more bearable than you think. It shouldn't weigh more than the original iPad, which was 681g, significantly lighter than the 12-inch MacBook at 916g. I wouldn't be surprised if it weighed as little as 600g.
Not a bad thing - it's interesting to think about. Apple could have some very interesting product overlaps: 12.9" iPad Pro/12" rMB and iPhone 6+/iPad Mini. And Apple has some very odd (in the sense that the rationale isn't really obvious) product rules, such as iOS won't support pointing devices (other than your finger