As in the question, do heavier riders tend to have higher/lower cadence and should there be any difference?
Individual "ideal cadence" is probably more closely related to muscle composition and comfort than it is to size. While someone could probably "maths out" an ideal cadence for a rider based on all the variables necessary (height, weight, power, muscle composition, etc), that won't matter one whit if the rider has spent the 2500 hours over the last 3 months practicing at a different cadence that they are now comfortable at. Riding is largely a practiced unconscious act (spinning) that is developed over time. Making major (or even minor) changes has the potential to ruin the whole unconscious part, making the act tedious and draining and overall less efficient for an individual. Developing the unconscious act of riding with pedal drills or cadence drills is a difficult task that most recreational riders don't have the time or patience for.
Muscle strength grows with the muscles cross section area. And the required forces to move a body grow with the weight of the body. Heavy persons must have the muscles to move themselves, especially if they are fit enough to enjoy bicycle riding. So, all else equal, a 120kg person should be able to exert significantly higher forces than a 60kg person. (I'm talking about the mildly overweight class of people at about 90kg to 130kg, not the extremely overweight class with more than 150kg.)
The cardiovascular system, however, does not scale with weight. The continued power output of a 120kg person is not significantly higher than that of the 60kg person. (Otherwise you would see much heavier riders at races as the speed of level riding is virtually independent of weight but very dependent on power output.)
So, the heavy person can exert more force, but cannot sustain more power. Power is force times cadence. As such, the heavy person is more comfortable with a lower cadence.