Thanks for the screenshot, Cressida.
It appears that it's shared files that are taking up your space.
You might want to use the steps here, to calculate the size of your folders; this should give you an idea of which ones are taking up the space.
If you want to leave a shared folder without affecting the other members, you can take a look here.
You can also check out this article on how to add some extra space to your account.
I hope this helps!