Plasma 5.7 so far has been adjusting the systray icon size with each release.
In 5.7.0 it moved to a much smaller size then was seen in 5.6 and previous releases, now with 5.7.1, it is back to a bigger size, for some way too big, see:
http://bugs.kde.org/365570
Future releases might receive an option to have the icon size user adjustable. For now the best option in KaOS is to use a simple patch for plasma-workspace. 5.7.1-2 is patched in such a way that small is back to being default.
To adjust to your preference, edit:
kdesu kate /usr/share/plasma/plasmoids/org.kde.plasma.private.systemtray/contents/config/main.xml
look for line 49 <default>1</default>
Adjust using number 1-6, 1 being the smallest, 6 very large. Save and restart plasma:
kquitapp5 plasmashell && plasmashell