In article <4b6n00$26bg@ns5-1.CC.Lehigh.EDU>, jx00@Lehigh.EDU says...
>Nikon lenses are considerablly bigger than other brands. For example, >the 70-210mm f/4-5.6 of the Nikkor AF lens is about one third bigger >than a Sigma equivalent. Does this mean Sigma has more advamced >technology than Nikon?

No. It means that Nikon makes fewer compromises with optical quality
than Sigma (though there are some good Sigma lenses - see "SUBJECTIVE
Lens Evaluations [Mostly Nikkors]" when the new version appears here
in a day or two). Bigger lenses can have more even illumination, better
edge sharpness, flatter fields, etc. more easily than smaller ones of the same type, though the larger glass costs more to grind (and the buyer may object more to the size, weight, and cost of the higher quality lenses). BTW, the Nikkor 70-210 f4 E and AF are about 1/2 again the size of the Nikkor 70-210 f4-5.6, and are higher yet in optical quality (though this may be coincidental - the f4 is 1 stop faster at 210mm than the f4-5.6).
Hope this helps.