Larger body mass (muscle) does not always equal greater strength.

FWIR, muscles are comprised of fast and sliw twitch muscles. Greater amounts of one over the other determines the type of strength you have.

Fast twitch is explosive movements (think squats, bench and deadlifts). Slow twitch are you endurance muscles (think long distance runner).

You then have people in between, who train for looks and symmetry. They are big and strong but not as strong as someone who purely trains fast twitch (and might actually be 'smaller')

Tl;dr

Think of cars. Some cars have big shells and others have small. Until you know about the engine, using physical shell size as a measure of speed is not always a good indication.