People with bigger hands usually have bigger wrists, elbow joints and joints in general, and therefore can put on more muscle mass/have more bone + muscle mass naturally and thus hit harder.
That's why people often talk about hand/wrist size as an indicator for the amount of strength an athlete can produce.
Muscle type (fast or slow twitching) plays a big, if not the biggest factor though, along with the leverage of a certain frame, which is down to bone length and where tendons and muscles are connected to bespoken bones.