Look at the size of priv.edb and pub.edb. The size of the files as reported by NT may be greater than the amount of data actually in the databases. Exchange runs a defragmentation process in the background. This process will result in compaction of the data without reducing the disk space used. Exchange will utilize the free space within the database file before the size of the file will increase. There is no way I'm aware of to determine how much free space exists within the database files.
By the way, I'm not sure what you mean about a 16GB limit. I'm not aware of any database size limitation in Exchange 5.5. But, then, I don't know everything...
Bob