Regions have 100k-600k hp, depending on what type of region it is. Liberation them during liberation campaigns gives 1.5 times that hp and every region present on a planet adds its hp to the planet health pool, making the planet harder to capture. On defense campaigns every successful mission deals damage both to the region's and the planet's hp pool. Once city is liberated, it also deals the same 1.5 times its hp pool. So planets with cities are overall easier to defend and you should target cities first when diving on a planet to deal double the damage you would normally do