Balrog getting killed?

never though about how he could do it, although i assume that he is a lot tougher then he lets on. although if we are goning to wonder about such things, how come the fall off the mountin killed the balrog, when both he and Gandalf survied the fall from the bridge? the fall from the bridge to the bottem of the pit that was reputated to be a much longer fall then any that could happen from the top of a mountin. kind of stange if you ask me