This was due to the fonts being sized incorrectly in older versions, which made them appear larger. With a number of character sets this resulted in the characters being chopped at the top or bottom. The newer versions of mIRC resolve this issue, however it means that you need to use slightly larger font settings to view the same size text :-)
Hello Khaled and thank you for your hard work on mIRC! To show my support I recently bought my own copy of mIRC
The font change you made in 6.33 isn't working for UTF-8 as it shrinks the non-ascii characters vertically (but not horizontally!) which makes the characters out of proportion and very hard to read even if you increase the font size.
Something causes the mIRC (6.34 & 6.33) to switch between the old and new font behaviour (of non-ascii text) so please note this shrinking problem is not active all the time.
If you cant figure out what causes this, please at least implement the old font behaviour as an option in the mIRC settings for the future mIRC versions, thank you.