I took the file $KDEDIR/share/apps/khtml/css/html4.css and changed it to make the buttons have a height of 16px and the fonts normal. I never really like the default BIG buttons --- and no web developers ever think set a size for the buttons because every 'popular' browser has the default behavior they want. I did this terribly and there are thousands of ways this can be fixed up. It will also violate the font sizes you have already set, so if it looks weird, just change the font sizes.
If you don't have access to root or don't want to replace that file, you can go to Konqueror>Settings>Configure Konqueror>Stylesheets>Use User-Defined Stylesheet