SQL Server memory management is designed to dynamically adjust the committed memory based on the amount of available memory on the system.

SQL Server uses CreateMemoryResourceNotification to create a memory resource notification object and SQL Server Resource monitor threads calls QueryMemoryResourceNotification every time it runs to identify if there is any notification. If a low memory notification comes from Windows, SQL Server scales down its memory usage.

How much it scales down?
Till “Min server memory”  (If there is continous memory pressure on the system).
What happens when you set Max server memory and min server memory to same value?
Ans:SQL Server will never scale down its memory usage even when there is memory pressure system wide (Lowphysicalmemory notification  set at system level)

What are the affects?
Ans:If LPM is not enabled SQL Server’s working set will be paged. If LPM is enabled system will starve for memory  and non-bpool will be paged.

Refer http://mssqlwiki.com/2012/06/27/a-significant-part-of-sql-server-process-memory-has-been-paged-out/ for more details.

Cap the SQL Server MAX Server Memory after considering the memory required by other applications, Operating system, Drivers , SQL Server Non- Bpool allocations etc. Make sure you have adequate available physical memory even when the system is under heavy load.

Make sure you have all the fixes for working set trim refer http://mssqlwiki.com/sqlwiki/sql-performance/windows-2008-and-windows-2008-r2-known-issues-related-to-working-set-memory/