In each pair, the countries’ capital cities have the same number of letters. The number of letters increases by 1 as you go down the list, from 4 in Rome/Lima to 10 in Washington/Bridgetown.
Congratulations 'ylj'......... so quick and a great explanation. I thought this challenge would last a little longer but 'jetpunkers' can never be underestimated...... well done and your 'Honorary Gold Medal' is on the way.