if you have a lot of contents, yu should have a google webmaster tool, from their you will find the answer you want. one reason that your indexed content get less in number is that many of the indexed content were taken down due to DMCA specially if your site is a warez site or something that can be considered as mere illegal.
You shouldnt have to worry about the indexed content of the site if you know exactly the number of content on your site. The numbers of indexed content in google cant be the basis of SEO standing of your site. Just compare the umber of content on your site with the number of indexed content by google, if the number google indexed content is lower than the number of content you have on the site, thats the time you should be worried for your page rank is falling