The problem that is happening is that you have scaled it simply by pressing <kbd>S </kbd> . 
This is happening because while doing rotation, moving or scaling , blender considers the `origin` of the object as the `pivot point` for doing all the manipulations.

 If you are wondering how the origin ( the one yellow point shown while selecting an object ) got away from the object's geometry you might have moved the object in the edit mode. **Remember that while in edit mode no matter whatever manipulations you do, the origin won't move along with the mesh as in edit mode**. To solve your problem you can simply scale it on the y axis by pressing<kbd>S </kbd> + <kbd>Y</kbd> .The proper way to do it is by going to **object > set origin > origin to geometry** and then scale it by just pressing <kbd>S </kbd>
[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/kw5Gx.png