The problem is that your object is scaled in object mode.

(like the cube in this example:)


[![enter image description here][1]][1]

Exit edit mode, select your object and press <kbd>Ctrl</kbd> <kbd>A</kbd> and select **Apply Scale**

[![enter image description here][2]][2]


[![enter image description here][3]][3]


  [1]: https://i.sstatic.net/Awbg4.png
  [2]: https://i.sstatic.net/2gIIx.png
  [3]: https://i.sstatic.net/u5COQ.png