The final mishna of this tractate teaches: one may not turn a field into a city's outskirts or vice-versa, nor a city's outskirts into a city or vice-versa. R. Eleazar says this applies only to the Levitical cities (which are specially designated). For Israelite cities you can "promote" -- turn a field into an outskirt or an outskirt into the city -- but cannot diminish, that they destroy not the cities of Israel. (33b)
(An "outskirt" is land adjacent to a city that is neither sown nor built upon, according to a footnote. Animals can graze there.)
I don't know how, or if, this applies to cities that grow together, like Yaffo and Tel Aviv (which is one city now).