How small animals sometimes beat larger rivals:
Specific traits that may provide advantages to small species in aggressive interactions included well-developed leg musculature and talons, enhanced flight acceleration and maneuverability and traits associated with aggression including testosterone and muscle development.
From “When David beats Goliath,” by Anne Craig at PhysOrg. The study was performed by Paul Martin of Queens University, Kingston, Ontario.
Should not surprise readers of this column.
Slightly East of New 
http://en.wikipedia.org/wiki/Mustelidae
The least weasel is not much larger than a mouse, while the giant otter can measure up to 2.4 m (7.9 ft)[citation needed] in length and sea otters can exceed 45 kg (99 lb) in weight. The wolverine can crush bones as thick as the femur of a moose to get at the marrow, and has been seen attempting to drive bears away from their kills.