This behavior is normal and is an intended behavior of the SQL Server buffer pool. By default, SQL Server dynamically grows and shrinks the size of its buffer pool (cache) depending on the physical memory load reported by the operating system. As long as enough memory is available to prevent paging (between 4 - 10 MB), the SQL Server buffer pool will continue to grow. As other processes on the same computer as SQL Server allocate memory, the SQL Server buffer manager will release memory as needed. SQL Server can free and acquire several megabytes of memory each second, allowing it to quickly adjust to memory allocation changes.
You can establish upper and lower limits to the amount of memory (buffer pool) used by the SQL Server database engine with the min server memory and max server memory configuration options. Before you set the min server memory and max server memory options, review the references in the section titled "Memory" in the following Microsoft Knowledge Base article: 319942 HOW TO: Determine Proper SQL Server Configuration Settings
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