É "normal"? Sim.
Você pode desfragmentá-los? Sim, mas você precisa de um utilitário MS adicional chamado "Contig" (abreviação de Contiguous).
Você pode obter o Contig daqui .
Tutorial ( aqui ):
Close the Outlook program.
Click the "Start" button, then click the "Search" box. Type "*.ost" in the "Search" box and press the "Enter" key. Right-click the "OST" file displayed in the search results and click "Properties" in the menu that appears.
Note the path to the "OST" file that is displayed in the "Properties" window that appears.
Click the "Start" button, then click "Programs." Click "Accessories" and then right-click on the command line icon. Click "Run as Administrator" in the menu that appears.
Type "cd c:\path_to_contig," substituting "c:\path_to_contig" with the actual path and directory where the Contig utility is located, then press the "Enter" key.
Type "contig --v path_to_ost/filename.ost," substituting "path_to_ost/filename.ost" with the actual path and name for the "OST" file, then press the "Enter" key.
Wait while the Contig utility processes the "OST" file. When the command line output confirms that the defrag operation has completed, you can start Outlook.
Mais informações aqui .
Espero que ajude ...