Jamal,When publishing a cached service, ArcGIS returns a warning message box when the estimated cache exceeds a given size. The 500MB you see is a default value which ArcMap sharing settings have been set for. This value is just something to make you aware of when you start caching perhaps because you have not planned to cache the map in all scales but forgot to deselect the certain ones. This value can be accessed from the ArcMap > Customize menu > Sharing tab. You can change the threshold for the warning by entering a new size. If you do not want the warning message to appear, uncheck Show warning when cache exceeds.The Estimated Cache Size estimate, displayed on the Caching tab of the Service Editor dialog box, is a very rough approximation based on average tile sizes for the image format you chose and the number of tiles expected in your cache. The estimate assumes a vector map for non-JPEG image formats and raster imagery for JPEG and MIXED formats. For JPEG and MIXED formats, the estimate assumes a compression quality of 75. Some deviation of the cache size is acceptable and this is something you should be aware of, so what you've shown on the screenshot is not wrong (several MBs difference is a very good estimation for ~800MB cache).When deriving estimates when your area of interest is a feature class, it???s important to note that the estimates only reflect rectangular map extents. If you specified a feature class as the area of interest for your cache, the estimate is based on the rectangular bounding box of each polygon in the feature class, not the exact feature class boundary. If you're going to cache an irregularly-shaped geographic feature, you may get a more accurate estimate by clipping a grid to the boundary of your feature class, effectively breaking up your feature into large grid cells. You can use the Create Fishnet tool to make a grid.
