You'd never know how much space you lost on your HDD for some created-once-and-forgot-to-delete kind of folders. Or you can just right click and choose "Properties" to check the size of each folders on your HDD. It is a damn tedious work.

Or maybe you can just try this

There is another similar software called Xinorbis. It's more advanced and hence, more complicated. I used to like it but then switched to Disk Space Fan.