Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Canon 400mm f5.6L USM is the longer of the two lenses at 257mm. The Canon 200mm f2L IS USM with a length of 208mm is 49mm shorter. On the other hand, the Canon 200mm f2L IS USM has a larger diameter of 128mm compared to Canon 400mm f5.6L USM's 90mm 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. Canon 400mm f5.6L USM weighs 1250g, which means it is 1270g (101%) lighter than the Canon 200mm f2L IS USM which has a weight of 2520g.
Below you can find the dimension and weight table of the 'Telephoto Prime Lenses in Canon EF Mount for size comparison purposes.
Telephoto Prime Lenses in Canon EF Mount for Size Comparison
Both Canon 400mm f5.6L USM and Canon 200mm f2L IS USM has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon 1D X III, Canon 6D MII and Canon 5D MIV.
Canon 400mm f5.6L USM is a prime lens with fixed focal lenght of 400mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 640mm.
Canon 200mm f2L IS USM is a prime lens with fixed focal lenght of 200mm. When it is mounted on an APS-C sensor camera with 1.6x crop, it provides an 35mm (FF) equivalent of 320mm.