"O şehrin nüfusu çok büyük değil."
Translation:That city's population is not very big.
Yes, the root is 'şehir'. But there is a rule such as that. It is called 'ünlü düşmesi' in Turkish, but i don't know the English grammatical equivalent of it.
The rule applies to some two-syllabled words having one of ı,i,u,ü in its second syllable. ıf such a word takes a suffix beginnig with a vowel the second vowel is omitted. Best way to learn them is practising.
burun-um = burnum (nose)
boyun-um = boynum (neck)
ağız-ım = ağzım (mouth)
alın-ım = alnım (forehead)
karın-ım = karnım (stomach)
isim-im = ismim (name)
fikir-imiz = fikrimiz (idea)
keşif-im = keşfim (exploration)
beyin-in = beynin (brain)
devir = devri (age/era)