Even though some fabrics are only woven in single width, especially some custom weaves, they are still weight estimated as if they were a linear yard of double width fabric, in order to make the system more consistent. So it would actually take two yards linear of single width fabric to determine the "weight" of that fabric, where it would only take one yard linear of double width to determine the weight of that fabric, since the standard is the wieght of a linear yard of double width fabric.

Also as Slohairt describes above the lighter the weight of fabric the more the tendency is for that fabric's double width to become even wider---16oz at 54" wide DW, 13oz at 56" DW, 10 oz at 60" DW--although not always. But it does help explain sometimes why a particular weaver's 13 or 16oz fabric does not feel like the same weight as another weaver's of the same alleged "weight". A 16oz per yard of 54" wide fabric should feel a bit heavier than a 16oz yard of 60" wide fabric, about 10% heavier to the hand actually. This can be particularly noticeable when you go from 16oz/yd 54"DW fabric to 13oz/yd 56 or 60" DW fabric, that the 13oz will feel even more substantially less heavy to the hand than the 16oz as its weight is distributed across a greater width of fabric.