Due to the extreme frictional heating from flying at a sustained speed of more than twice the speed of sound, Concorde would expand in length by as much as 9 or 10 inches because of thermal expansion. Concorde the best aircraft to ever grace the skies expanded due to the kinetic heating on the airframe. Concorde would fly so fast that the skin heated up causing the air frame to expand. This was most noticeable on the flight engineers seat whereby a gap would open up over the course of the flight. The gap was so big you could put your hand in. When the aircraft slowed down it cooled down and the gap closed. A good example showing this is the hat that was placed in the gap on one of the last flights meaning it is now stuck in there.
Second reason is to with differential pressure and the expansion is not as much. Modern aircraft fly at altitudes around 35,000 and to allow passengers to survive the flight they are pressurised to somewhere around 6000ft. This means the aircraft skin is effectively pushed outwards and expands slightly and any dimples in the metal are squeezed out. These pressurisation cycles are the main factor that can limit the service life of an aircraft. Pressure cycles cause metal fatigue over time and this must be closely monitored.
By – Krister Sundelin & John Mack