Originally posted by Alcazar i think they are going for small and light...
k-s1 vs k500 size comparison
The difference in size on the grip is not as much as it looks by these pictures.
K-500 has a big part around the shutter button that is deeper and wider than the actual grip, and the K-500 body is deeper so the front grip is not much deeper than on K-S1.
The biggest difference is probably the position of the shutter button.
The K-S1 might have more comfortable grip than K-500, and it might be easier to reach buttons on the back with you thumb.