I am upgrading a .NET application from ArcGIS 9.3 to ArcGIS 10.3. I've noticed that the map control behaves differently: the map no longer automatically rescales itself when the window is resized. To put it another way, when you resize the map the scale stays the same even though the extent changes. I would rather that the map rescaled when the window is resized, as it did under ArcGIS 9.3.
The commentary below from ESRI explains that this change is by design.
Less redrawing in data view
In previous versions of ArcMap, if you changed the size of the ArcMap display while working in data view, either by resizing the ArcMap window or by docking/undocking/resizing a dockable window, by default your map was completely redrawn to fit inside the available display area. So the scale changed and the extent stayed the same (although you may have seen some extra geographic coverage based on how well the extent fit inside the new shape of the display area). At version 10, the default drawing behavior in data view has been changed so that when the display size is changed, your map is no longer completely redrawn to fit inside the display. Instead, the scale stays the same and the extent will change. If you make the display bigger, you'll see a larger geographic extent, and vice versa.
This has the performance advantage that the portion of the display unaffected by resizing doesn't need to be redrawn at all. For example, if you close a docked window, only the portion of the display that was obscured by the window needs to be redrawn. It is also easier to work with the display because geographic features on your map don't move around as you dock and undock windows. Features on your map remain in the same location in the display until you manually pan or zoom the map.
In summary: I want the ArcGIS 10 map in my .NET application to rescale when the window is resized, just as it did under ArcGIS 9.3.