SQL Server Express 2012 Database Size Limit

SQL Server Express only imposes file size limits on data files, log files can grow to any size. The Size shown on the database properties window is the data and log file combined sizes.

Somewhat confusingly though, the Free Space value shown on this window only relates to the data file(s). If you'd like to see this in more detail, I'd recommend querying the system tables rather than relying on the SSMS GUI (which isn't always going to give you the information you want/need). The following script will show some more detailed information about data file sizes/growth settings:

SELECT 
    [TYPE] = DF.TYPE_DESC
    ,[FILE_Name] = DF.name
    --,[FILEGROUP_NAME] = FG.name
    ,[File_Location] = DF.PHYSICAL_NAME
    ,[FILESIZE_MB] = CONVERT(DECIMAL(10,2),DF.SIZE/128.0)
    ,[USEDSPACE_MB] = CONVERT(DECIMAL(10,2),DF.SIZE/128.0 - ((SIZE/128.0) - CAST(FILEPROPERTY(DF.NAME, 'SPACEUSED') AS INT)/128.0))
    ,[FREESPACE_MB] = CONVERT(DECIMAL(10,2),DF.SIZE/128.0 - CAST(FILEPROPERTY(DF.NAME, 'SPACEUSED') AS INT)/128.0)
    ,[FREESPACE_%] = CONVERT(DECIMAL(10,2),((DF.SIZE/128.0 - CAST(FILEPROPERTY(DF.NAME, 'SPACEUSED') AS INT)/128.0)/(DF.SIZE/128.0))*100)
    ,[AutoGrow] = 'By ' + CASE is_percent_growth WHEN 0 THEN CAST(growth/128 AS VARCHAR(10)) + ' MB -' 
        WHEN 1 THEN CAST(growth AS VARCHAR(10)) + '% -' ELSE '' END 
        + CASE max_size WHEN 0 THEN 'DISABLED' WHEN -1 THEN ' Unrestricted' 
            ELSE ' Restricted to ' + CAST(max_size/(128*1024) AS VARCHAR(10)) + ' GB' END 
        + CASE is_percent_growth WHEN 1 THEN ' [WARNING: Autogrowth by percent]' ELSE '' END
FROM sys.database_files DF
LEFT JOIN sys.filegroups FG
ON DF.data_space_id = FG.data_space_id 
order by DF.TYPE desc, DF.NAME; 

As an aside: Since your data file is at 10GB already, future growth operations will fail (and whatever transaction triggered the growth will generate a Primary Filegroup Full error). Since the file is set to 10GB exactly, I suspect that it has been grown out to this amount deliberately to take advantage of the full amount of space offered by SQL Express.

SQL Server will allocate more disk space as your database grows. You may start with a 1GB but it will grow to 3GB over time. Since it is inefficient to re allocate for each record, it grows by x% or xMB (you can define these). The available space is how much empty disk space you can use before the next growth.

If for some reason you belive your databse had grown into unrealistic size, you can use DBCC SHRINKDATABASE(<YourDatabaseName>)

As for the express limit, I'm not sure exactly how it is calculated. But it is possible that most of your database size is the log file, especially if you have done lots of updates and deletes and you haven't done any backup.

You can check the log file size:

DBCC SQLPERF (LOGSPACE)

If you discover that the logfile is indeed oversized and you don't need the option for point in time recovery, you can change the recovery model to simple and that will reduce it's growth significantly

Recovery models: https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/recovery-models-sql-server