Relative window dimensions of child windows are unaffected by the relative size of their parents - They are sized relatively to the render window. This is not explicitly stated in the documentation, which misleadingly states that the size is relative to the parent. The Window functions for minSize, maxSize and size are affected by this. In my opinion the common sense and most intuitive effect of the size of a child window would be to relate to the parent window.
For example a parent window (which is attached to root) having a unifiedWidth of 0.8 and its child having 0.2 should result in a total size of renderwindowWidth * 0.8 * 0.2, effectively inheriting the size to the children. This is common in any scenegraphs and node systems and seems more logical to me.