Eu testei em atop-2.3.0-8.fc27.x86_64
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RSIZE, não PSIZE, é usado ao ordenar por uso de memória (use a tecla M ), e para a coluna MEM.
RSIZE PSIZE MEM
311.0M 260.2M 4%
303.3M 288.1M 4%
217.6M 123.1M 3%
Saída mais detalhada:
PID TID MINFLT MAJFLT VSTEXT VSLIBS VDATA VSTACK VSIZE RSIZE PSIZE VGROW RGROW SWAPSZ RUID EUID MEM CMD
1782 - 2413 0 16K 96480K 361.5M 288K 3.8G 311.0M 260.2M 10772K 9448K 0K alan alan 4% gnome-shell
1455 - 0 0 280K 30860K 340.8M 132K 1.0G 303.3M 288.1M 0K 0K 0K root root 4% packagekitd
13561 - 41988 34 196K 143.2M 298.0M 136K 2.2G 217.6M 123.1M 255.3M 139.0M 0K alan alan 3% Web Content
13333 - 25219 0 196K 119.6M 277.9M 144K 2.1G 190.9M 111.9M 0K 3560K 0K alan alan 2% Web Content
The memory percentage (column MEM) shows the resident memory occupation by this process, because that is what matters when your system starts swapping.
The memory details (subcommand 'm') show the current RSS per process in the column RSIZE and (as a percentage of the total memory installed) in the column MEM.