Search engines don’t index every page of your site every time they crawl through your pages, unless you have a very small site. The size of your crawl budget is determined by the quality of your site. For Google, PageRank has the most significant effect, followed by the amount of organic search traffic.
The PageRank of your site is, essentially, a quality score assigned by Google. Sites start at 0 and then work their way up as their website grows in authority. PageRank is determined by the number of other pages that link to a page, and the quality of the pages doing the linking. A link from a highly authoritative site will help your page rank more than a link from a brand new site.
The higher your PageRank is, the more time Google will spend indexing your site. A site with a PR value of 1 or 2 will see a smaller number of pages indexed than a site with a PR value of 7 or 8. Because you can’t change your PageRank (at least, not immediately), your goal needs to be to make the most of the crawl budget you currently have.