All SQL Server data modifications (updates, inserts, and deletes) are initially recorded in the database log file. Depending on the frequency of modification and your backup and recovery model, the size of the database log file can grow to large proportions in a short amount of time.
Use sp_helpdb to determine, among other things, the size of both data and log files separately. The stored procedure returns two separate recordsets.
EXECUTE sp_helpdb MyDatabase