Translation:Istanbul has grown so much while I was not here.
because the speaker "has come to know" after Istanbul has grown. That's why speaker talks like he heard it from someone else. In Turkish, the suffix "-miş" is commonly used for the events/processes we heard/saw after it is happened. -miş is also used for something you don't completely understand something at first, but after thinking thorougly about it.
‘(y)ken’ expresses contemporaneity with the main clause, independently of the tense of the latter. It is equivalent to the English ‘while + -ing’, (except that it can express a different subject), e.g. ‘yürürken konuşuyorum’ = ‘I'm talking while walking’, ‘yürürken konuşacağım’ = ‘I will talk while walking’; as you can see, both in English and in Turkish the tense of ‘walking’ doesn't change (although it is implied to take place at two different times in the two sentences).