Created attachment 136612 [details]
Example of big icon with panel height at 38
I noticed that in Plasma 5.21 using the panel with the height set at 38 results in the panel icons being bigger than before. This also mean the icon is now very close to the highlight effect's line. When reducing the panel height to 36, the icon sizes are reduced and there is more vertical padding.
As Nate said, "seems like the icon sizes jump to the next biggest size a little bit too early to feel comfortable".
STEPS TO REPRODUCE
1. Compare icon sizes when alternating between two panel heights: 36 and 38
Fairly big icons with panel height at 38
Normal sized icons with panel height at 36
KDE Plasma Version: 5.21.2
KDE Frameworks Version: 5.79
Qt Version: 5.15.2
Can reproduce with a horizontal panel with sizes 38 and 54.
A possibly relevant merge request was started @ https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/237
Fixed by Niccolò Venerandi with https://invent.kde.org/frameworks/plasma-framework/-/commit/b4e4d21219d753882b4bb226a15647ab23796cd6 in Frameworks 5.82!