If you use pidgin, you'll notice that pidgin have issues in most window managers (gnome, cinnamon, <put something more here>). I see this issue a lot of times in pidgin's issue tracker but i don't know exactly who did it wrong, but since I'm able to fix it easily....
After searching for a while, I find this (quite old and still an issue). So the problem is the hicolor icon theme... ffs. Let's fix this (atm, it's quite different to fix it, but it works):
1) Go to this folder: /usr/share/pixmaps/pidgin/tray/hicolor
2) Make a backup or just rename your 16x16 to something else (16x16.old will do it)
3) Link or copy your 22x22 to a new 16x16 folder
4) Restart your WM
It's quite an odd fix, but it works (until someone fix it properly and I have OCD with the problematic icon).