"It has developed into a very large city."
Translation:C'est devenu une très grande ville.
"C'/ce" is an indefinite pronoun with prompts a masculine singular adjective (by default).
Only "Elle (la ville) est devenue" would need an agreement in feminine.
Surely this translates as ‘it has become a very large city’. DL accepted my ‘il s'est développé en une très grande ville’ but to me ‘developed into’ suggests a deliberate planning act whereas ‘become’ could be accidental and so they have different meanings. I think DL should be clearer.
"To develop into" is fine in English, but "se développer en" is awkard. As usual, the original sentence was written in French and its best "natural" English translation happens to be better with "developped into".
This is one of the limits of "natural" translations which do not translate back to the original sentence exactly.