A lot of the "hunk" and weight in the camera comes from needing a certain amount of room for the mirror and sensor, plus a solid coupling mechanism for the lens

A lot of the "hunk" and weight in the camera comes from needing a certain amount of room for the mirror and sensor, plus a solid coupling mechanism for the lenses, plus a good amount of surface to provide comfortable grip, plus a number of slots for all kinds of connectors (for your tripod, for you external flash), etc. I'm not saying the design can't be improved, or that every empty recess couldn't be filled with extra battery capacity, but personally I find the current state of affairs (where you can just get an extra battery for $20 and throw it in your camera bag) better than a phone-like model with non-replaceable batteries.