Does ideal cadence depend on a rider's weight/size at all?
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.