That's probably because some "big" font setting, either in IE (options) or in Windows (fonts DPI).
Thanks for pointing me to that direction.
It's not the fault of IE or DPI settings. I manually increase the font sizes of several elements of window (from 8 to 10).
With size 8 everything is back to normal.
I have to live with it, since I will not change the size only for Avast.