Expansion due to heat, the GP mentions a temperature of 300c.
Edit to add:
Different materials expand by different amounts due to heat, and different parts of the aircraft are reaching different temperatures.
If you know all the bits are going to expand x amount in normal flight, then you work back to what size they are when cold, if the shrinkage/expansion is too great you possibly could go with a different material (with various tradeoffs) or you just accept the shrinkage as they did here.
Edit to add: Different materials expand by different amounts due to heat, and different parts of the aircraft are reaching different temperatures.
If you know all the bits are going to expand x amount in normal flight, then you work back to what size they are when cold, if the shrinkage/expansion is too great you possibly could go with a different material (with various tradeoffs) or you just accept the shrinkage as they did here.