This is true no matter how large the lens barrel is and so the lenses will collect the same amount of light given equal scenes with the same shutter speed and ISO. There are a The larger front element doesn't translate directly to a faster aperture -- since they're both rated at f/1.4, they theoretically collect the same amount of light.At least in most tests I've seen, it appears that the Sigma does vignette less than the Nikon though.
The nutshell answer then is that it doesn't really.
I'll leave the rest of my answer to explain what does matter...
If you are concerned about image quality and handling, you should be looking at specifications like the maximum aperture, something like an MTF chart, sample photos from the lenses, or other reviews.
No, a designation of f/1.4 implies the same ratio between focal length and aperture in both lenses.
If you're shooting a crop digital body, this is moot as you're not going to get the corners where vignetting is visible. Also, I'd like to point out one reason not being mentioned -- sensor (or film) size.
Nikon DX lenses (for instance) are designed for their 1.5x crop d SLR's, so basically, when you mount one of those on one of their full-frame d SLR's, you will get vignetting in the edges/corners.Essentially a larger diameter allows the lens to be designed with a larger maximum aperture.More light means shorter shutter speed which means less motion blur when holding the camera directly, IS or not.I have taken a few shots with one, and found it quite impressive -- but I have no idea whether that's typical, or my friend just happened to get a particularly good copy.None of the pictures I took would really qualify as serious stress testing either, so while it did well, under the same circumstances, I'd be a bit surprised at anything doing particularly badly.It also retains relatively round out-of-focus highlights toward the edge of the frame, where most of the competitors start to get fairly elliptical toward the edges. When DPReview tested it, they found it distinctly better than either the Nikon or Canon counterparts.