This is a bit long for a comment so I'm putting it here.
First try running this:
exec sp_msforeachtable 'sp_spaceused [?]'
Yes I realize it uses sp_msforeachtable and there are a number of better methods, this is just quick and hopefully "good enough".
At a guess you have a heap (no clustered index) that has a large amount of "unused" space. I wrote about how to duplicate it here. If in the list of tables you find one that has a large amount of space shown in the
unused column then try running this:
ALTER TABLE tablename REBUILD
Fair warning it may take up some space to run the operation but it should free up the excess space in the table.