I had a similar problem about a month ago, I just accepted my MacBook was full, until I went through all my root folders and using the 'Get Info' option.
It gave me a size of each folder, so I just added them up and found I was using about half the space it said I had.
Turns out, when I performed an Archive and Install, I forgot to delete the archived system, so I had effectively doubled my Data
Maybe you have done the same thing?