When you resize (but only decreasing size) the control, the position and size of the Marker indicator is not updated.
This is reproducible with RadVerticalLinearGauge and VerticalLinearScale. When the RelativeHeight of the scale is set to 1 and the RelativeY is set to 0.
As a work around you can avoid setting the RelativeHeight and RelativeY properties of VerticalLinearScale.