That is the way SQL Server is designed to use. The SQL Server caches as much data from the disk into memory as the data is being used so that it has faster access to it.
If you connect to the SQL Server with Management Studio (regular or Express) and use sp_configure you can set the maximum amount of memory that the SQL Server will be allowed to use.
EXEC sp_configure 'max server memory', 1024
Now with SQL Express you have the upside that it can only use 2 Gigs of RAM, so it won’t be able to use any more than that.