It's amazing how many options there are for lens designers. Look at this, these four lenses are the same focal length and the same aperture size. And yet there are huge differences in size.
The size influences thing like sharpness and vignetting. It's much harder to make a small lens (given the same sensor size) than to make a larger one.
The one on the left is brand new, the first new fast Normal lens in many years. It's a Sigma.
I really like zoom lenses, but I also like choice and compactness, so I appload Sigma (and Pentax) for being rank-breakers and actually produce new, high-end prime lenses (non-zoom) designed for digital.
From the review:
"The most striking aspect of this comparison is the sheer size of the new Sigma; it's distinctly bigger than the Canon (previously the largest in this class), and simply dwarfs the petite Pentax. This is fundamentally a reflection of the monstrous optical unit in the Sigma, which is designed to minimize vignetting at wide apertures; the front element is fully 60mm in diameter, compared to less than 40mm on the other lenses. The relative weights reflect these sizes (the Sigma tips the scales at 505g, more than twice the Pentax's 220g) and in turn the prices correlate with those weights; in this case you're literally paying for all that glass."