Slartibart's answer is spot on. I'll just suggest that, as the first step, open the Photos library, click Photos > Preferences, and in the General tab, see whether "Importing: Copy items to the Photos library" is checked or not:
- If checked, the photos will be imported, and will take twice as much space (X GB in folders, X GB in the Photos library).
- If unchecked, Photos app will only create references (pointers) to the photos in your folders and not actually import them into the library. This won't take much extra space but you shouldn't move the folders or the photos within them, otherwise you'll have broken links.