In the Process tab you typically see either Memory (Private Working Set) or Commit Size.
Private Working Set is the amount of physical memory used for the process.
Commit Size is the amount of virtual memory guaranteed to be allocated for the process. This does not mean the pages are present in physical memory, and does not even mean the memory was actually allocated.
The Performance Tab shows either the sum of the commit sizes, or the sum of the allocated sizes next to the memory bar. Under heavy load that would be significantly larger than a combined private working sets of all processes, meaning that part of the pages were swapped out.
