Probably as much good fortune as planning, the 4/3 sensor size happens to be optimal for a small but capable system, given current sensor and lens technology.
Anything larger, like APS, and the glass gets real big, while the performance increase isn't noticable in typical shooting and displayed via current methods.
Anything smaller like One, and you start seeing losses in typical photos, plus a noticable deficit in DOF control. As well, with current lenses and current control technology, making a body smaller than an EM5 or G/GH results in handling issues, especially with longer lenses, and external control issues, such as body too small for the controls that are desired - thumbwheels, dedicated buttons, etc...
This can change. Sensor technology, and bokeh in software can bring the One sensor up to parity with 4/3, in most situations. Changes in lens technology, especially 'liquid lenses', can further reduce the size and layout of lenses. Different control layouts can bring greater functionality to small bodies.
We have yet to see an interchangeable lens system for cell phone cams, and that's just a matter of time. Someone experienced in small optics, like Olympus, should partner with Apple to develop this - a series of ultra compact AF lenses that could fit a Photo iPhone. Could even add a small case to the bottom of the phone to store a UWA and a mild tele lens, they'd be about as thick as a pencil. Add the phone's big screen and controls plus extensive software options... that could be a real winner.