Sexlinks are a hybrid strain of fowl bred particularly for eggs, to my understanding. Cornish and Buckeyes are different, buckeyes only slightly. Nether breed will lay quite as well from what I've observed.
As far the the pelvic width of female vs male.....based on my experience, you can't compare the two....a wide pelvic width on a male only means that he is fat. I breed males that have a longer breast bone.....this prevents males that are short in back and coupled up front. This also prevents males that tend to narrow in width down he body. A buckeye needs to be of equal width from back to tail and many have become to wide up front and narrow to the back. This is a result of breeding overly for a wide chest. This trait inturn is passed to the female offspring and allows for greater egg production. This trait also allows for breast meat capabilities in future generations.