1/4 inch vs. 3.5 mm Jack Difference? - Head-Fi.org - Headphone forums and reviews for audiophiles
I guess originally the 1/4" (6.35mm) jack was the standard jack, then Sony came out with the 1/8" (3.5mm, mini-jack) for the first "Walkman". As the 1/4" was not practical for portable audio. I believe studios and "professional audio stuff" sticks to 1/4"...