Why does septoplasty ALWAYS make your nose wider?

Septoplasty (like the name implies) is an operation on the septum.  The septum is INSIDE the nose.  Septoplasty operations should NOT make a difference outside.  If you see that a patient looks different after septoplasty, it's because sometime additional to the septoplasty was likely done by the surgeon and the patient.  Most often, what is done by ENT doctors is the "shave down" the nose hump at the time of the septoplasty.  Unfortunatley, if this is the only technique that is used to treat the numb, the nose WILL look wider if external steps are taken.

   A septoplasty by itself will not widen the nose.  There are other techniques that are utilized to help with breathing that will widen the nose.Kenneth Hughes, MDLos Angeles, CA

A septoplasty is a functional operation and has no effect on the with of the nose. Temporary swelling might make it appear so but is this resolves the nose will assume it's preoperative shape.

A septoplasty alone will not alter the shape or appearance of the nose.  A septoplasty in conjunction with a rhinoplasty will change the appearance, but whether this change results in a "fatter" nose will depend on the patient, the surgeon, and what the patient and surgeon were trying to achieve.  It's certainly possible to have a smaller, yet stronger nose that allows you to breathe better.  Consulting in-person with 2-3 rhinoplasty specialists will give you a much better idea of what changes are possible to your nose.  

A standard septoplasty performed through the nostril (the closed approach) often does not change the appearance of the nose.  If the lower two-thirds of the nose is crooked, a septoplasty may straighten the nose. If a nose is appearing wider after a septoplasty, the patient is either having a concurrent rhinoplasty or they are having overresection of the septum causing the tip to drop and appear wider on the frontal view. 

A septoplasty alone does not affect the width of the nose. A septoplasty with spreader grafts CAN widen the nose slightly. Other maneuvers that might have been done at the same time may well cause a wider nose or just an appearance of a wider nose.

If dorsal spreader grafts are placed along side the septum to improve nasal valve collapse or to straighten the septum, it can add about 1-1.5mm to each side of the septum.  It depends on how high the grafts need to be placed. If these grafts are not used, the septum stays the same.  Not everyone needs them, but many do.  I have added a link below.  Not all the noses are wider post op and all have these grafts.