Kcalc Version 2.7
Using KDE Development Platform 4.5.5 (KDE 4.5.5)
John Culleton
you may like to try a userspace workaround :-)
In Kcalc go to menu and pad Settings/ConfigureKCalc/Font/[Button Font]
then change ythe font and/or size and/or boldness as you like
(suggested "Liberation Sans" "Terminus" "Luxi Mono" etc.