OpenTTD City size declining

I had a city with 25000 citizens, but it started to decline. As you can see on the screenshot there are a lot of empty spaces. There used to be building along those roads.

To prevent the decline I added bus routes within the city. According to the OpenTTD wiki that's a way to increase town sizes:

A bus with one passenger from the other side of town would have the same effect on growth as a large train full of passengers from the other end of the map. source: wiki.openttd.org

But the city still shrinks. Does somebody know how to prevent this?

It looks like your bus stop is on the city's central tile - the tile directly under the city name. This tile is the city's "seed tile" - the city grows outwards from there. When a player-owned building such as a bus stop, train station etc is placed on the seed tile, the city is unable to spawn new buildings and will slowly die off.

Try moving the bus stop to an adjacent square and see if the city starts to regrow again.