"Power" can also be situational. The hero may have a whole Marine Amphibious Brigade on his side and the terrrorist just a few dozen fanatical followers, but if the villain is striking with terrorist bombings from a hidden base, the villain is more situationally powerful -- at least until his base is located, and even then, the villain might have hostages, or a secret escape tunnel, or the whole thing could be a trick to lure the Marine Amphibious Brigade into a labyrinth of booby-traps, etc.

The "sympathy for the underdog" effect is of course why apologists for villains and villainous causes like to emphasize how much stronger the heroes are. We saw this in the Vietnam War, and we have seen this in every war America's fought since then. It's important to remember that, despite this human tendency, being an "underdog" doesn't really mean being a good guy.