In the browser, Dropbox tell me I am "using 3.19 GB of 2.75 GB storage". My PC tells me I am using 26.4 MB. I've compared the files and folders in Dropbox with what is in the Dropbox folder on my PC and they seem to match. I have also permanently deleted all deleted files in Dropbox. As best as I can tell, the usage numbers in the browser are wrong.
I was well over quota and deleted a number of large files and ensured that the sync is complete.