"Upgauging also saves on engine maintenance and airport fees. There are just powerful incentives to try and get more seats per flights."
To pack more passengers in many airlines have added added extra rows on to aircraft.
For example, in 2007 Southwest Airlines' 737 aircraft would carry 137 passengers but now the aircraft have been reconfigured to fit 143 passengers.
JetBlue is reported to be adding two extra rows to its A320 aircraft, increasing passenger numbers from 150 to 162.
Also, in the mid-1990s, standard seating on a 777 aircraft was nine seats across and it is now not uncommon to see 10 seats in a row.
In addition, much larger aircraft have been introduced to the market such as the jumbo A380 aircraft, which is able to carry 853 people in a single class cabin.
But the economics of the A380 have proved daunting, with airlines having to operate every flight at full capacity in order to make a profit and the plane only being able to land at certain airports.
Earlier this year, Airbus was close to scrapping production of the A380 until Dubai's Emirates airline made a large order for new models.