See yesterday’s discussion in Micro Four Thirds: Withering on the Vine, plus Micro Four Thirds vs APS-C: Size and Weight Realities, or search for Micro Four Thirds.
Reader Herve L points out that image aspect ratio might be a deciding factor for some users. DSLR cameras have 3:2 sensors, but M4/3 has a 4:3 aspect ratio (which also helps in keeping lens size down, square format being most efficient at this).
I am not a proponent of any fixed aspect ratio (what a silly argument), having shot 4X 5, 6X17 and 6X7 and 35mm and so on. Each has its merits, but I am used to the 3:2 aspect ratio of a DSLR, which is also the same for APS-C and the Sony RX100 oddball size. I think it works best in most cases. And with modern printing, I crop to whatever feels right—rare but no hangups there.
Micro Four Thirds is a 4:3 aspect ratio. If one wishes to make 3:2 images, then the M4/3 sensor is effectively even smaller as it will have to be cropped. On the other hand a 20 X 16 print has to be cropped from a DSLR, but that is why I would make a 24 X 16.
Either way, making a fixed demand about the aspect ratio does influence the sensor size difference: cropping APS-C to 4:3 yields a 20.9 X 15.7 mm sensor, which is 46% larger than M4/3: still significant.