How often is "usually"? Is it 20% of the time? 60%? 80%? 90%?
Did you see this New Yorker article on how the underdog can win? http://www.newyorker.com/reporting/2009/05/11/090511fa_fact_...
An interesting quote from that article:
What happened, Arreguín-Toft wondered, when the underdogs likewise acknowledged their weakness and chose an unconventional strategy? He went back and re-analyzed his data. In those cases, David’s winning percentage went from 28.5 to 63.6. When underdogs choose not to play by Goliath’s rules, they win, Arreguín-Toft concluded, even when everything we think we know about power says they shouldn’t.