"Did the population of the city decrease?"

irregular and there is no rule

Some words, such as şehir, lose the vowel before the final letter (or it is transferred to the end of the word) when they are part of a possessive construction. My city=benim şehrim; your city=şehrin; his/her city=şehri, etc. Sorry, I can't think of the other Turkish words like this, off the top of my head. Will add more if I do. As I recall, the words are from Arabic.