If I remember correctly, the decoy volume treats all the hidden space as available disk space. TrueCrypt used to have a warning that booting into the decoy volume could scramble the hidden volume when the OS wrote files to disk if it happened to choose some space that overlapped with your data.
If your decoy only lists 5GB of space on a 5TB drive, then it isn't a very good decoy.
If your decoy only lists 5GB of space on a 5TB drive, then it isn't a very good decoy.