SSD drives are pretty expensive, so most of the modern computers (both PC and laptops) comes with a smaller size SSD and an HDD (if required). The idea is to install Windows (or any other OS that you might be using) and in the SSD, which is designated as the primary boot drive (or the system drive), and majority of the programs in the HDD.
However, there are certain rogue programs which doesn't let you choose their installation path and end up getting installed in the SSD drive. Slowly, over time, the SSD drive becomes full and you have to upgrade to a bigger and more expensive SSD. Google Chrome is one of those rogue programs.