The problem is that **your object has been scaled in object mode, but the transformation has not been applied**.

Take this perfectly regular cube (2mx2mx2m) for example:


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


Look at the **scale** section on the panel:

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

Switch to **Object Mode**, select your object and press <kbd>Ctrl</kbd> <kbd>A</kbd> and select **Apply Scale**


[![enter image description here][4]][4]


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

[![enter image description here][5]][5]


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