So, if you never back up an SQL Server it uses space endlessly as the transaction logs grow and grow. Have you tried backing it up? A backup causes a checkpoint, which lets SQL server know it doesn't need to keep certain transaction log files.
You might need to back it up to a remote system if the drive is too full. Remember, the backup will be written by the Windows account that SQL is running as, not your user account - so that user needs to have access to the network share you are attempting to back up to.
For your reference:
There are other reasons for MSSQL file growth, of course, including insertion of data. ;) If that's not it however, you can look at things like transactions that aren't ever committed.
The answer to your stated question about changing from MSSQL to some other platform is that it depends. If you are referring to an app that already supports multiple DB back ends, then it can be as easy as dumping your data to some universal format, then importing it in to the db platform you prefer, then switching your app to use your preferred DB - but in most cases it is not this simple. In most cases, the application is tightly coupled to the RDBMS platform and it's not as simple to "change back ends" without re-writing significant parts of the code.