Originally posted by mikesbike Olympus uses a smaller sensor, so the comparison is really not fair. We don't seriously compare the K-1 with the KP for size without taking the format into account. A better comparison is with mirrorless designs having APS-C sensors. And then what about a built-in flash, control features, the OVF and its quality, etc? And yes, there is such a thing as an overly small design degrading handling and ergonomics. An optional battery grip can address some of these issues, if available.
It can be fair to compare the two especially if you compare the kp to the em1. There is only a .5 crop factor difference between the two, even smaller if you compare m43 to a canon apsc body (.4 in that case). I havent used the kp, but from what i hear it punches above its weight class, so does the em1. Low light would go towards the kp, af speed the em1 wins. Both have great image quality in good light, the m43 on average will have smaller lenses (not always the case, but to the most part).
To the op on camera size, there is a main difference between the em10 and the em1. Id place the em1 in overall size similar to the k3 just not as thick (not all m43 cameras are small). The em10 on the other hands is going to be very small and compact for an ilc and more then likely much smaller then the kp (think coat pocket size). The kp will have the better features in comparison to the em10, especially with weather sealing where the em10 has none. The em1 on the other hand has more to it, decent weather sealing, af speeds faster, but from what i hear a more complicated menu system then the kp.