SQL Server will grab and keep all memory that you let it. If you don't want it to use 15.3 GB of memory, you need to change the setting so it only grabs X GB.
You can do this by right clicking the instance in the object explorer, click on Server Properties, and change the Maximum server memory under the Memory tab. It is generally a good idea to leave at lest 1 - 2 GB for the operating system, and then more if you have anything else running on the server (you should avoid running other stuff on the server, if possible).
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