With the latest patch, Dota 2 seems to have a new bug where it auto zooms in and then try as you may, you cannot zoom out no matter how much you scroll your mouse-wheel. Here is a fix!
Firstly, you will need to enable the console. You can do this by right-clicking Dota 2 in your Steam library, then choose Properties > General > Set launch Options. Add this line to the launch options: -console and thereafter you can open the console by pressing the tilde key ~ whilst in game.
Now to fix the zoom, enter this command into the console and press enter: dota_camera_disable_zoom "1"
This will reset the zoom to the normal position and prevent you from zooming in or out again.