Their share of overall population grew from 18.2% in 1999 to 23.1% in 2009. Their population increased, on average, by 3.1% a year between 1999 and 2009. This was much higher than the average growth rate of metropolitan areas in OECD countries (0.86%) during the same period. In contrast, the average annual growth rate of population in medium-size and small FUAs was only slightly higher than in OECD countries during the same period.