I want to know why it is using up so much space and why its contents are hidden?
I'm thinking that is your trash can. If you open nautilus, right click the trash bin, and hit empty trash, its size should substantially decrease.
Of course, I'm assuming it's a folder. Am I right, or is it a regular file?
Less than a minute of googling later, I believe my answer is right. Optionally, delete it from the command line. But first, educate yourself and read through the top few links in this google search: