By default Swift Performance generate global CSS files, which is good, because it will using less space, and also takes advantage of the browser caching.
However in some cases, styles can contain dynamic parts (mostly inline styles). This can cause issues, because Swift Performance will use the hash of the CSS file as a file name, and when CSS will be regenerated, and the content is a bit different, it will produce a different hash – and a different file.
After a time your cache folder will be filled with abandoned CSS files, because Swift Performance can delete global CSS files only when all cache has been cleared.
You can exclude problematic styles from being merge, or alternatively you can use Separate Styles feature.
If you enable this feature Swift Performance will generate separate CSS for all cached pages. It can increase a bit the cache size, however it won’t produce abandoned CSS files, so in long term cache size will be smaller.