With film and manual focus plus manual aperture, there was little in the cameras. Manual film transport too meant all you needed was a tiny battery to power the exposure system, at least that is how I used my dads Tlb canon. And it still works.
Move to a T90 and you get multiple motors for film transport, more buttons and less dials. But to do this you have more electronics which go somewhere. Size goes up.
Move to a digital canon and you have to add in all those things peculiar to digital but absent with film. Rear screen, memory card(s), bigger battery, flash, dials to zoom in or change focus points and so on. You then have to have the buttons spaced such that they are usable.
Following a logical set of steps you can see how dslr bodies range from the 1 series to the rebels. My only criticism of the 1series is the weight! But even that can help with balance or unsteady hands. My 350d seemed to produce many more crooked horizons which are absent (mostly) on the 1dx.
The now decades-old Canon film 1-series SLRs that used the EF mount were roughly the same size as modern 1-series EOS DSLRs. The size of the 1-series has nothing to do with electronics. It is an ergonomics factor...always has been. The large 1-series have always had the integrated grip...its one of its selling points. The large size offers much better hand-holdability in both landscape and portrait orientations.
That is, has always been, and probably will always be one of the primary arguments for using an SLR-style camera...ergonomics. Even if you have smallish hands, mirrorless cameras are exceptionally small. They really don't fit the hands all that well, and while they certainly save on weight, for all but those with tiny hands, they end up having other ergonomically related problems, like cramping your hands into your face in an uncomfortable way.
From a weight standpoint, one of the most significant weight factors in a modern DSLR is the battery. Larger batteries with much greater capacity are usually the most dense aspect of the camera. A large battery can get you a lot more shooting time before having to swap out batteries, and/or supply the necessary power to move the mirror and actuate the shutter and write huge amounts of data to a memory card at extremely high frame rates. It's a trade off you can choose to make, however...1D X with grip and extra large battery, or 5D III with no grip and a relatively small/light battery. Perhaps at some point in the future dense, heavy batteries might be traded out for some kind of light weight fuel cell that can operate for days under the same kind of load conditions.