After quite a search I found the reason why the icons in my system tray (kubuntu 14.04.4) got so big from time to time. I am 100% certain I found the reason.

Normally it looks like this: (look below picture for the rest)



But when I substitute all systemfonts by the beautiful andica-ttf font, the system tray looks like this:



The icons in the system tray are about as big as the aplication launchers. I find that ugly. I tried in desktop effects substituting "system" by "raster" and switched between opengl 2 and 3.1. It all didn't help.
I hope somebody knows a way to use the andika font and still have a nice system tray.
Only when I don't use the andica font the system tray looks normal again.

I found a fix:

There's another workaround (I wouldn't call it a fix).

Edit the file /usr/share/kde4/apps/plasma/packages/org.kde.systemtray/contents/ui/main.qml
and change the line:

property int iconSize: Math.min(root_item.width, Math.min(root_item.height, theme.defaultFont.mSize.height < 20 ? 24 : theme.largeIconSize)) //Math.min(root_item.width, Math.min(root_item.height, JS.ICONS_SIZE))

by the line:

property int iconSize: Math.min(root_item.width, Math.min(root_item.height, JS.ICONS_SIZE)) But only until next upgrade, as this file will be overwritten...

As far as I can see now it works. I don't understand anything of it. But I hope it can help other people.