Keep in mind, it's still a rebel with rebel size and ergonomics. To me, a lot of the reasons to shoot with a pro camera or even a prosumer is just the better feel in the hand, buttons, controls... aka ergonomics. Things that you can't list on a spec sheet. Mounting any real glass on a rebel feels incredibly unbalanced to the system as a whole.
Total shot in the dark here, but could the different between the two be a traditional DSLR and a mirrorless? One of them removes the mirror and adds an EVF? With how conservative Canon is, this could be their way of hedging a bit and seeing how the market responds. Give the consumer the choice in basically identical bodies and see which one is popular? A little experiment, test the waters, and let it help drive their future market decisions.
I think its certainly possible to have a camera of a similar kind of size to the current standard rebels and also offer improved handling and build though, maybe not to quite the degree of the 70D but still an improvement.
When it comes to mirrorless I think its notable that recently we've actually seen bodies starting to grow in size(T-X1, E-M1, GH3/4, NX1). With those cameras your really no longer talking about something significantly smaller/lighter than a rebel, what your talking about is a camera with superior handling/build, that's a market that an upgraded Rebel sized body could also aimed for.