I apparently found the plugin that is causing the larger boldface letters — CSSHero.
CSSHero is my last plugin installed, so it was the first one I reactivated and saw the boldface fonts go larger. I just started learning CSSHero and have used it very little. (Perhaps, though, I did something to cause the problem).
After discovered the CSSHero problem, I stopped the debugging procedure, deactivated CSSHero again and continued building my site.
Should I reactive the other plugins now?