Well, there's a lot of ways to look at this - this wouldn't result in more pages indexed, so the two issues are totally separate.
If the goal is to get more pages indexed, then a site map (either XML or event a text list) uploaded to your server for Google to find can help. Or, at least that makes sure that Google is finding and indexing the pages you want them to find. Your Google Webmaster Tools account (assuming you have one) will also tell you some data.
For example, we used to have 100K+ pages; many weren't quality content I wanted to rank. Like, a PDF of a catalog ranking about the product page. So, I reduced the number of pages indexed so Google would have better, more quality content to serve to searchers.
Using Xenu or Screaming Frog is another good way to help uncover pages. Those tools crawl your site like Google would,then you can download the file and not only see all the URLs found, but also if they are 301/404/200, etc. And, Screaming Frog can crawl your site and output a XML sitemap for you (it's an easier way to make one).
I prefer SF and it's about $150 US dollars for the use - well worth it.
As for why - well, if you have a lot of pages, Google doesn't always find them. That's where a site map can help (it directs Google what to crawl). Otherwise, there could be technical issues to a bunch of pages and they aren't properly linked up or something and that could be causing the issue.