I am using the Xfce system-load panel plugin (v.1.1.2), -- package default for Xubuntu 16.04. I have the panel set to the left side of my desktop (not top or bottom). All the graphs shown in the sidebar adjust in width when the panel size is changed, EXCEPT the memory graph. The memory graph does not change with the panel width. Please fix this bug.
All 3 bars adjusts width for me when I change vertical panel width with xfce4-systemload-plugin-1.2.0-1.fc25.x86_64 and xfce4-panel-4.12.0-5.fc24.x86_64. Possibly you might want to upgrade and retest?
Please test with 1.2.1, it should be fixed.