It's not true @Mafklapper78, with my respects. I wanted bigger map too, so I came here, searching for a solution.
as @MayorBig mentioned perfectly, it takes too much process to render all the details, vehicles and pedestrian movements. After playing some hours on a city, it's on edge of 200K people. But the problem is I can't play smoothly on my core i5-9400 and GTX 1070 8GB with 32GBs of ram.
Now I get it!!! It is a regional city building game. If you want single bigger city, play cities: skylines instead. it will be smooth untill 300k inhabitants with far more larger playable area. But disasters there... are really gigantic!!! it's not skylines... it's all about getting ready for huge disasters, and ultimately a huge Tsunami will be the final day of your play on a map