Database Cache Size Resident (MB) is the amount of system memory (in MegaBytes) used by the database cache that is currently ...

Database Cache Size Resident (MB) is the amount of system memory (in MegaBytes) used by the database cache that is currently part of the working set of the process.  If Database Cache Size Resident (MB) is ever significantly smaller than Database Cache Size (MB) then the operating system has chosen to reclaim that system memory for use in other parts of the system.  The database cache will recover from this event but if this is a common occurrence then it can lead to significant performance problems.