Quite often photographer's are misled to believe that a camera's crop sensor increases the "reach" or magnification of an image. This is not so. The camera's sensor density has more to do with the magnification than the crop factor. This is only when photographing a subject.
This however is not true for video. The sensor's crop factor does make a difference since most camera manufacturer's average the pixels across the entire sensor.
In this video, three cameras are tested. Each with different crop factors. Canon's 5D Mark III has no crop factor, while the Canon 7D has a 1.6x crop factor and the Panasonic Lumix GH3 has a crop factor of 2x.
All three cameras were mounted on a Canon 600mm II f/4 lens set at the same distance to a suet bird feeder. (Special thanks to Downy Woodpeckers used as a model.)
Besides all three crop factors, the Panasonic GH3 has a built-in teleconverter function that uses the center pixels of the sensor, magnifying the video by another factor of 2x. In this mode, any slight vibration does cause issues since there is no image stabilization available when marrying the Panasonic GH3 to the Canon 600mm lens. In this sample, the wind played havoc with this mode. Resolution however is still increased 2 fold.