@Rick M. That's correct. If a user with a new, empty 100 GB Dropbox accepts a 99GB shared folder, they're fine. They have 1 GB left on their quota. Once the shared folder reaches 100 GB that user will no longer be able to add files; however, other users of the share CAN continue to add files, potentially increasing it's size past 100 GB. It's possible for a Dropbox to be "full" but actually be "over-full" because of a massive shared folder.
If a user has a 100 GB Dropbox with 25 GB personal data and 50 GB shared folder, and someone adds 49 GB to the shared folder then the total size is now 124 GB. The user will need to erase more than 24 GB of their personal data before it'll stop showing them as over-quota.
It's had and tough finding out no matter how many files you delete, you are still stuck with same size.
In the end, I figured out Dropbox was right!
I had to use TreeSize to peek my Dropbox folder
(It is very likely people are using Selective Sync which can get the situation more frustrating - I used selective sync, which means, not all my dropbox folders are sync onto my machine, hence my Dropbox folder property will be showing incorrect info since the remaining folder are not synced and are still on the cloud)
TreeSize software showed me some folders that were unexpectedly large, I wouldnt have thought they could be that large.
TreeSize software also showed me the dropboxcache folder needs to be monitored - it was large beyond my believe consisting of files used by the Dropbox process.
In the end, the patience required by to figure this out can be solved by the Dropbox team altogether if there was a way to check size of everything inside dropbox including folders and files, and I think the dropboxcache is synced which I dont expect to be.
The dropboxcache folder is not synced/doesn't count against your Dropbox space, but it is used by the client as part of the sync process. That said, erasing the cache folder is a solution to some syncing issues which can occur. If erasing it helped, then you might have had something going on besides just being low on space.
There are several limits on space: your hard drive and your dropbox quota to name two. Shared-folders count against your dropbox quota. If you have ANY shared folders THEY will use more of your space up to the point where either your hard drive is full or your dropbox quota is exceeded. Use Selective Sync to stop loading files you do not need.
Is this still an issue for you? If so, are you deleting files from your local Dropbox folder, or directly from your online login? If it’s from your local Dropbox folder, then it may take a bit of time for the deletions to register online too.
Like @Trinity C.1 said, have a look at your Shared folders, as when you accept an invitation to a shared folder, it is added to your Dropbox too.