I tested on atop-2.3.0-8.fc27.x86_64
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RSIZE, not PSIZE, is used when sorting by memory usage (use the M key), and for the MEM column.
RSIZE PSIZE MEM
311.0M 260.2M 4%
303.3M 288.1M 4%
217.6M 123.1M 3%
More detailed output:
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
https://www.atoptool.nl/screenshots.php
The memory percentage (column MEM) shows the resident memory occupation by this process, because that is what matters when your system starts swapping.
https://www.atoptool.nl/download/case_leakage.pdf
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.