Hi Kristina,
Thank you very much for taking the time to review our site.
These 15000 pages were basically Layered Navigation pages. We did not feel that they were useful for Google, so we used to block them through robots.txt. However the Google bots got them somehow into their index. So then we added canonical tags on our pages and removed the block from robots.txt (In June 2015). Finally Google bots removed those pages from the index due to the canonical tags. We have around 2000 "real" pages on our website. Therefore right now it seems to us that Google's Index is showing the right number of pages. This has been the case since Sept 2015, but our rankings have stayed low.