1. Go to "Tools" -> "Options..." -> "Build & "Run".
2. Open "Compilers" tab and look at the size of the QTreeView control.
3. Switch to "Qt Versions" tab and look at the size of the QTreeView control.
   The control's height gets smaller.
4. Go back to "Compilers" tab.
5. Switch to "Kits" tab and look at the size of the QTreeView control.
   The control's width gets smaller. The buttons in turn get wider.

All these tabs show nearly the same controls. They all have buttons and one big QTreeView. For a consistent look and feel the QTreeView as well as the buttons should always have the same size.