Thanks to the folks at Olympus, I get to test the Olympus OM-D E-M1 Mark II and OM-D E-M1X.
Both are called “micro four-third” sensor cameras. That means the sensor is 17.4 mm on the long side by 13.0 mm on the short side. Contrast this with a “full sensor camera” that has a sensor that is 35mm on the long side by 24mm on the short side. (Notice the sensor is the same size as a 35mm piece of film.)
The advantage of a “micro four-thirds” sensor is things appear closer.
The Canon Rebel T6i has a “cropped” sensor, or 22.3 mm by 14.9 mm, so the same object appears farther away.
The Canon 5D Mark IV is a “full frame” camera with a sensor 35mm x 24mm. Objects appear much farther away.
I took each photo above from the same place. Each camera had a 300mm lens with a 1.4x tele-extender. That means I was using a 420mm lens for each photo but the subject was more or less magnified based on the sensor size.
The Olympus “four-thirds” sensor would mean a bird would be larger in my photo. The “four-thirds” sensor would mean I might not have to crop as much since the subject would already be bigger in the photo.