I think crocs will try and take a smaller hipo. Usually, the herd will set up a barrier of bodies to protect their young. I did remeber a PBS special on a killer croc. This thing was huge. I think it killed several villagers. They had film of it and the hipos were backing up. They tried to capture it. I think they called it 'Gustav'.
http://en.wikipedia.org/wiki/Gustave_(crocodile)
Tom