...
There are two concepts that must be strictly related, that and those are the camera sensor size (or film back size) and the render resolution, and or more precisely the sensor size aspect ratio (the quotient of the horizontal film sensor size by the vertical film sensor size) and the render aspect ratio (the quotient of the render horizontal resolution by the vertical resolution).
In a real camera, the image size is a direct consequence of the sensor size itself, and so the image aspect ratio and the sensor aspect ratio are obviously equal .
But in a 3D program, you can freely are free to have a render aspect ratio that differs from the camera sensor aspect ratio, which is incongruous for a physically correct camera. As Because Maxwell works like a real camera, it forces the camera sensor to fit the render aspect ratio, causing that you can get a different framing in the render than what you have in your platform viewport.
Just check that your current camera sensor aspect ratio (by dividing the horizontal film sensor size by the vertical film sensor size, accessible from that camera Properties panel), is equal to the render aspect ratio (dividing the render horizontal resolution by the vertical resolution).
...