I've hit this recently and have been trying to delete enough stuff to make space - the google drive website is awful for doing this. So bad that I wonder whether it's deliberate. I've got 14.8GB of data on there and can't work out where it is.
On my computer, the sync'd Google Drive files come to less than one GB, so it's not that.
It doesn't show folders but clicking the text "14.8 of 15 GB used" or this link[0] will take you to a list of the biggest files, at least in drive. You can also go here[1] to see what services are using the most data.
Thanks. I came back to add the same info, found by googling, that the storage amount includes other Google services. (my space is mostly taken up by photos + gmail, because I've cleaned out google drive in an attempt to get the usage down, not realising that it wasn't the issue. Looks like I'll be spending hours painstakingly deleting photos and emails...
On my computer, the sync'd Google Drive files come to less than one GB, so it's not that.