The Silverlight layout system reads properties within the SizeChangedEventArgs argument class of this event, to determine whether the reported size changes should be considered significant. This allows the layout system to avoid forcing a layout change due to visually imperceptible differences between old and new height or width values. The imperceptible differences might be due to rounding or calculations of a floating-point value.
The SizeChanged event uses specialized event data derived from RoutedEventArgs, not EventArgs. However, the event does not bubble (does not travel a true event route).