This is normal behavior and nothing to be concerned with. Exchange includes a feature called the Dynamic Buffer Allocation. This allows the buffer cache to grow or shrink as needed, depending on how much RAM is available and on what resources are in use. If other applications are not using memory, the Exchange Server Information Store (store.exe) consumes up as much memory as it needs. If other applications need memory, the Store gives up some memory by transferring pages to the hard disk and shrinking the size of the buffer. I recommend leaving things as is since this is the intended behavior. Limiting the amount of memory available to Exchange (via PerfOpt) may end up causing problems down the road.
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