Answer
When exporting to .jpg (or any raster format) from ArcGIS 9.0, it is possible to set the export resolution in dots per inch (dpi). As the dpi increases the dots in the x direction (width) and in the y direction (height) also increase.
If the application selected to view the file is set to view .jpg files at a specific resolution, the .jpgs are displayed at that same resolution, regardless of the export dpi. Higher dpi files display at a larger physical size than the same file exported at a lower dpi.