MaxMemoryUsed

gives the maximum number of bytes used to store all data for the current Wolfram System session.

gives the maximum number of bytes used during the evaluation of expr.

Details

On most computer systems, MaxMemoryUsed[] will give results close to those obtained from external process status requests.
MaxMemoryUsed[] will not typically account for code space, stack space, or the effects of heap fragmentation.

Examples

Show the maximum memory used during a session:

See how much the maximum memory used increased for a computation: