There could be multiple reasons why SQL Server is consuming lot of physical memory (RAM).

Following are the few that I would like to list out:

1) Performance of the server could be bad as the queries that are hitting the sever are not optimal.
You can further fine tune such queries.

2) Maximum and Minimum server memory configuration is not set up properly.
You can set up maximum server memory configuration so that it doesnt eat up whole of the RAM.

Following are the Steps to configure sql server memory allocation:

• Connect to the SQL Server in SQL Server management studio
• Right click on the server and click on properties
• Now the Server Properties window pops up.
  In the Server Properties window Select Memory.
  Now check the values of
  Minimum Server Memory(in MB) and Maximum Server Memory(in MB)

  

• Adjust these values to opimal memory values as per your SQL Server’s memory requirement.

Note: Be aware that changing the memory configuration of SQL Server might give you very bad results. Hence be cautious before making such changes.
You might also get There is insufficient system memory in resource pool ‘default’ to run this query error if memory configuration is bad.