Manufacturers sometimes make the sensor smaller. This is still a great quality camera, it's just as not of as high a quality as a 'full frame' sensor camera You will notice that the smaller sensor does not reach all the way to the edge like the larger sensor does. What this means is that when you attach the same lens to either camera, the camera does not see through that lens in the same way. In the full frame sensor camera, you will see more of the scene than in the smaller sensor.