I heard somewhere that a recent Supergirl was actually strongr than Superman
When Kara showed up on Earth, she was shown as having more punching power than Superman... but they said that it was because Clark knows restraint, and only hits as hard as he has to (doesn't want to cause collateral damage, or get anyone hurt). Kara just hit with everything she had everytime. I believe they make it pretty clear that if Supes just hauled off and used everything he had, he would be the stronger of the two.