Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Panasonic 100-400mm F4.0-6.3 is the longer of the two lenses at 172mm. The Panasonic 100-300mm F4-5.6 II OIS with a length of 126mm is 46mm shorter. Besides being longer, the Panasonic 100-400mm F4.0-6.3 also has a larger diameter of 83mm compared to Panasonic 100-300mm F4-5.6 II OIS's 74mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Panasonic 100-300mm F4-5.6 II OIS weighs 520g, which means it is 465g (89%) lighter than the Panasonic 100-400mm F4.0-6.3 which has a weight of 985g.
Below you can find the dimension and weight table of the 'Telephoto Zoom Lenses in Micro Four Thirds Mount for size comparison purposes.
Telephoto Zoom Lenses in Micro Four Thirds Mount for Size Comparison
Lens Mounts
Both Panasonic 100-300mm F4-5.6 II OIS and Panasonic 100-400mm F4.0-6.3 has the same Micro Four Thirds lens mount. Some of the latest released cameras that are compatible with the Micro Four Thirds lenses are Panasonic GH6, OM System OM-1 and Panasonic GH5 II.
Focal Range
Panasonic 100-300mm F4-5.6 II OIS has a focal range of 100-300mm and 3.0X zoom ratio which has a 35mm equivalent focal range of 200-600mm when used on a FourThirds format camera. Panasonic 100-400mm F4.0-6.3 has a focal range of 100-400mm and 4.0X zoom ratio which has a 35mm equivalent focal range of 200-800mm when used on a FourThirds format camera.