This is a bit of a debatable subject.
While everyone will agree gill curl is caused by bad water and or temperature shock, not everyone will agree to the fish turning sharply causing gill curl from a smaller than what is considered normal tank.
Many people in Asia keep arowanas in very small tanks by US standards for a very long time. I personally know a guy in singapore who has a 23" 24k gold keep in a 80 gallon for 15 years and till this day the fish is fine. There are people who keep them in 18'' wide tanks and they are healthy. Many people in china keep them in 16-18" wide tanks and the fish are fine.
I can also understand that not all fish have the same temperament and some may stress themselves out due to the space around them regardless if the water and tempertaure are fine.
I have keep adult asian, australian and south american silver arowanas in 3x2x2 and 4x2x2 for over a year some 2 years at a time before they moved into their new tanks, and the fish have been fine.
I personally believe that water quality and tempertaure stability are the most important factors for keeping all arowana healthy, but in some fish the size of the tank can also have an effect on it's health.