Why would you want this?
First, a narrower back frame on a shower commode chair may be required for individuals who are slim. Since an 18” base frame is the minimum width for fitting over a toilet, narrowing both the base and back frames is not an option for those who require over-toilet use. By narrowing the back frame only, the arms are positioned more medially and the back upholstery is sized more appropriately. Typically, 16” or 14” widths are ordered.
Some clients may have wide upper bodies and slimmer hips. For these situations, a 2” wider-than-standard back frame can be ordered.
Alternatively, arm spacers of either ½” or 1” per side can be added to space the armrests outward.
Link: Bathing and Toileting