It is actually the seat width rather than cabin width which is relevant to an individual pax.
For example the A320 is 6"(15cm) wider than the 737/757, which is normally translated into a 1" wider seat, to the advantage of the pax. However some airlines (Northwest for one) put the same seats in their A320 they do in their 757s. Therefore 6" wider aisle and/or more shoulder-room for the windowseat passenger, but no extra seat width.
The Trident (727-style from the 1960s) had one operator, Channel Airways, who put 7-across seats in it. It just depends how narrow you want to make the seats.
Martinair, Netherlands charter carrier, put an interesting config in their first DC-10s in the 1970s, 3-4.5-2. The 4.5 in the centre section was made with a reduced width "child" seat right in the centre. Didn't experience it but there were pictures in the aviation press at the time. This must have been a nightmare to administer the seating allocations for, does anyone know how they handled it ?
The "some seats are better than others" zealots will also notice that, where tray tables are done armrest-stowed rather than drop-down (eg in row 1) the extra armrest width required is just being removed from the seat width allowed. So bulkhead seats can mean more legroom but less width.