In the new Supergirl No. 2 comic, Supergirl seems to be shown as being stronger than Superman.
No explanation was given for that.
This scenario has been tried before, the last time Supergirl rebooted. Even then, no real explanation was given.
I just don't care for a slim Supergirl being stronger than a much heavier, taller and muscular Superman, so DC had better make this only a temporary situation.
IT's OK for the new Supergirl to have an attitude, but not greater powers.
UPDATE: Even at Comic Con 2012, there were references to Supergirl's superior super abilities..