First rule of hiring an SEO firm, don't hire a firm that reaches out to you randomly and tells you that you have something broken and they can fix it. FYI, I also know a prince in Nigeria who needs access to your bank account and would be happy to pay you for it.
That said, the whole pages on site vs pages ranking in Google is a valid measure. If most of the pages of your site are not ranked in Google, then that may be an indicator that you have a lot of low quality pages and this could be an issue. This type of metric is a good one for check in the health of your site.
That said, your have to figure out if this matters to your site or not and it is pretty easy to do. Various ways to look at this.
Log into Search Console and if you have a sitemap submitted on your main dashboard for sitemaps you will see URLs submitted vs URLs indexed. At a basic level, how many pages did you show to Google and then how many pages did Google say, yes, these are good enough to be indexed.
Use ScreamingFrog or the MozCrawl, see how many URLs are found by crawling your website. How does this total compare to what you show in your sitemap? Do you have a bunch of pages that are just tag cloud pages or a page that is just a resorted or printer friendly version (aka duplicate) of another page? You probably need to block access to those (and make sure they are not in your sitemap).
ScreamingFrog has a API to connect to your GA data. You can then also pull what type of search traffic do these pages get. This gets back to your ranking question. If you have a page that Google is ranking for something, it should get some search traffic to it. Depending on how much traffic you get overall to your site, you may need to look at a 3 to 6 month time period. You can now say, out of all of the URLs on your site, if there is a group of URLs that is not getting any traffic from Google for the past 6 months, they are probably not ranking for any key words. Why is that? Look at those pages. Are they crappy pages with very little or duplicate content? Updating a meta tag will not do anything if you have a crappy page to start with. Maybe you need to noindex them or 404 them and get them off your site. Maybe you need to add content, write more, include some graphics etc. Maybe you have duplicate or similar pages you need to combine into one super page and redirect the other page to it.
Good luck!