OK, got some new thoughts about it:
The option to show artwork only helps, but I was always curious why the art of MB is shown with the window frames - it would look so much nicer without those frames and the artwork could be much bigger!
I'm unable to make a printscreen of task switcher window, however recently I started to use BetterDesktopTool (useful app for managing windows and desktops) and in it's windows switcher MB looks bad:
On the left bottom you can see MB's window - but it's really tiny and almost unvisible.
In the upper middle you can also notice that I had MB's prefs opened - and this window is showed properly (if anyone asked - withoud prefs window opened MB's window is tiny as well).
Can anything be done about it?