SQL Server taking a lot of memory
SQL Server is known for by default consuming as much memory as the OS allows it to take, so you'll have to reduce it manually.

One way of doing this is described here and here. On the second link, specifically, you will find this example that might be useful for you:

sp_configure 'show advanced options', 1;
GO
RECONFIGURE;
GO
sp_configure 'max server memory', 4096;
GO
RECONFIGURE;
GO


Instead of 4096, you might want to enter something smaller depending on your available RAM.

And, as @outcoldman says in a comment to your question, SQL Server Express doesn't allow 15 GB databases. This answer specifies 4 GB max for SQL Server Express 2005 and 10 GB max for SQL Server Express 2008 &amp; 2012.