Traveller, maybe we're talking at cross purposes, in fact I've always ended agreeing that it is likely possible to further shrink a Rebel body, I've even edited the last post while you were typing to make this as clear as possible. But maybe not. I do feel the OM series is more compact than rebels, that's doubtless, it's a fact. But, at present, an APS-C DSLR can't be as compact as a film OM or the OM-D, no way. You provided correct measurements for the flange-focal plane distances of the old OM system (46 mm) and for EF/ EF-S (44 mm, in fact you can mount Zuiko lenses on EOS cameras with a cheap adapter 2mm thick), but you don't provide the distance for the OM-D, which is 20 mm, if I remember well. Therefore, in any case, at present the answer is that unfortunately it's NOT possible to engineer an EF/EF-S DSLR to make it as compact as, or comparable to, the OM-D. 24mm difference is roughly 1 inch, you can't put aside this fact. Don't feel offended, I insisted to make this clear, not to hurt you in any way.
I said the OM-D is micro 4/3.
I've owned an OM 2n since 1980, then I added an OM 1n and an OM 3, many many Zuiko lenses and, over time, an insane number of accessories for the Olympus system. I've been shooting with them for 30+ years. I went for the Olympus system primarily because of the size and weight. I still have all of them, all in perfect order, I occasionally use them (actually the OM 2n, my preferred one), and I think I know them quite well. Still, I think you can't squeeze all the stuff needed for digital in an OM-sized body currently. Just as an example, an LP-E6 batt alone is about as thick as the OM body itself.
We're talking at cross purposes here, so I'm not going to continue down this road (I've culled the reply-counter reply chain, as it was getting ridiculous!). You know better than most how small a 35mm camera can be made; do you not feel that the OM series feel more compact than the "Digital Rebels"? I can't see any reason why it would not be possible to make a DSLR this small (i.e. roughly comparible in size to the micro 4/3rds OMD EM-5). I can understand your concern about the battery size, but I can't see that this would be any more of a problem with a DSLR than with a mirrorless body. In fact, DSLRs tend to be less power hungry because they're not constantly running the sensor - LCD/EVF to provide viewfinding capabilities.
You're still missing my point!
The comparison between the Rebel and the OM-D was made because I could find side by side comparison photos of them and I could also find side by side shots comparing the OM1 to the OM-D, but I couldn't find any shots comparing the Rebel to the OM1. If you can find the latter, or are willing to photograph them yourself (assuming you own a Rebel), I would be most grateful if you could share the photo.
When you compare the OM1 to the OM-D EM5 (sidenote: why not just call it the OM-D1?), the additional thickness caused by the mirrorbox is not all that great. The bizzare thing is that small differences in overall dimensions don't seem to be as important in determining bulkiness as other design factors. The OM1 and OM-D EM5 aren't that much smaller than the Rebel, but they both appear to be so (to me at least) because of their cleaner lines. With prime lenses and careful design DSLRs needn't be anything like as bulky a system as they currently are.
Perhaps Canon could make a smaller DSLR as a modular system (like the OM1 popularised back in the '70s), or even a modular EOS-M series with hand/battery grip options and something like the Sony LAEA2 adaptor (with moving mirror and a pentaprism -go on Canon, take up the challenge!). This latter option would be a good way of enticing EOS system users to buy into the EOS-M system as well: use EF-M lenses without the adaptor when you need maximum compactness, or swap in the adaptor, battery grip and EF(-S) mount lenses when you need performance (the elephant in the room is full-frame and whether Sony will make a full frame NEX stills camera...)