It is normal for a transaction log to grow, especially if you have that many users accessing it and that much data stored in the database, however 15GB is rather large and you probably want that space back.

It all depends on what is more important to you, how well the SQL Server database is backed up or how much space the transaction log is allowed to use. You obviously have a good backup plan in place, so the real question is, can you afford to lose data up to your last backup? If you're backing up every night, the real question is, can you afford to lose a day's worth of data? If you answered no, leave the transaction log the way it is. If you answered, yes, change the transaction log's recovery type from Full to Simple. This change will instantly shrink your transaction log file to just a few megabytes, and you won't see the file grow to 15GB again.

EDIT: To answer your original question, there's no set ratio of how big a database should be in relation to it's transaction log. The transaction log just keeps a record of everything that's gone on with the database for backup/restoration purposes. In short, interaction with the DB grows the transaction log.