Apparently, Mac is not able to recognize and categorize the entire files in the correct groupings, because of that some of your files are considered system files by it.
WHERE CAN I FOUND THESE BIG FILES:
First thing that you can do is to look at the file composition
About this Mac (apple logo on the top left) > Storage
A tip for you: The size of those colorful blocs may not be true size. There are parts of it which Mac cannot recognize, is grouped with "system files" by your Mac.
In my case, it's the Mobile Sync files (iPhone Backups) and some my photo (or pictures) backups, which are not classified as pictures. I have 60GB pictures in invidual files and photos library, but in the file composition, it only says I have 30GB pics. Where does those 30GB pics go? Yes... it is considered by Mac as system files.
Another Tip:
To more accurately pinpoint the location of these files you need to look at your files composition in more detailed way.
Open your Terminal and type these:
cd / [enter]
sudo du -hd 1 [enter]
and then type these, after that:
cd ~ [enter]
sudo du -hd 1 [enter]
Mac may ask you for login password.
Those commands above will show you the main folders in your hard drive and their sizes in more detail. So now, you can compare it with the indicators in About This Mac storage groupings. This terminal command doesn't show you what are inside the folders. This information will give you indicators of what folders take most space, OR what folders that DO NOT MATCH the storage composition in About This Mac groupings.
You don't have to be confuse with the long name of these strings, just look at the end of of each line, each string will name the app / program it is related to. You may see Safari, iChat, news, stocks, etc.
I locate my mobile sync and photo folder location and size by this method. They are not classified as backup files nor photos. So what does Mac think about these file? You are right, those files are treated as system files.
GOOD NEWS FOR YOU
Good News: Don't worry, it is not a virus or malware or bug. Your system is ok. What takes up your storage is your real own files.
Clean up or move your files, especially your old pics, phone backup, and clear some cache. That's all you need. Safe your money from buying cleaning apps.
If the About This Mac still shows big system files, just ignore it.
I hope Apple can address this issue in the future by showing the exact amount of space and identify the files correctly according to its category.
Don't worry about your iCloud storage. About This Mac can identify its size correctly. It's not about your file in iCloud. Your mission in solving this problem is to identify what files that are not correctly grouped by Mac into its major groupings (documents, photos, itunes, books, icloud, mail). If Mac doesn't recognize the file, it categorizes it as System.
