Exchange 2013 Memory usage very low

Hi,

we have 4 Servers, 2 Mailbox Servers and 2 Database SErvers with DAG deployed on Exchange 2013 CU5.

We have about 420 GB in one database.

The Server has 40 GB Memory assigned (Windows VM), but only using about 12 GB of Memory.

Depending on this guide, this makes sense:

http://blogs.technet.com/b/samdrey/archive/2014/02/03/exchange-2013-memory-ram-allocation-by-the-store-with-an-example.aspx

But, as this is only a database server with no other services on it, how can i increase the memory consumed by Exchange?

User are reporting of slow Outlook Clients.