Since the release of 2.6.1, keepassxc doesn't use the set theme's 16x16 panel icon:

But rather uses its own icon which is far too big:

2.6.1A smaller icon or the ability to choose one.
A tray icon that's too big
KeePassXC - Version 2.6.1
Revision: 9a35bba
Operating System: Linux
Desktop Env: awesomewm
Windowing System: X11
This is expected behaviour and we're not going to change it.
you have to admit, the tray icon is quite jarring compared to other qt applications