A size comparison of the 400mm f/3.5 ED~IF, the 300mm f/2.8 AFS-II,and the 300mm f/4 AFS. As you can see, the 400 is more slender than the 300 2.8 or the 300/4 throughout most of the barrel. The 400 actually handles better on a monopod than the 300/2.8 does. Size-wise, it's easy to see why the 300mm f/4 AF-S is such a popular lens--it's relatively "small".