I have been watching videos about ligers, crosses between male lions and female tigers. They grow to enormous size, over 1,000 pounds, almost twice the size of either parent. Yet it is the majority opinion that they would not survive in the wild, because they are so large they would be unable to procure sufficient prey to maintain their bulk. With the increased size comes too much of a decrease in speed, agility, and stamina.
This drives home the point of diminishing returns with regard to size and combat, one fans of the UFC have witnessed for years. Despite our instincts to the contrary, huge men (over 300 pounds) fair poorly against slightly smaller men and lose consistently to them, as you can see here. They simply cannot maneuver their huge mass enough to defend themselves. With humans, or nature, there is such a thing as too large.