Lenses have a natural cos^3 or cos^4 falloff with respect to angle and because a given film size subtends a greater angle with a shorter focal length, there will be more falloff than with a longer focal length. Different lens designs lead to different amounts of this natural falloff.
Originally Posted by SMBooth
It's got nothing to do with coverage, which is generally where mechanical vignetting kicks in and gives you a distinct image circle. And likewise, a faster lens does not necessarily have greater coverage at all - tele lenses (where the rear nodal point is well forward of the rear of the lens) generally have less coverage than you might expect from their focal length.