What determines what is a "cleat"?
Nothing. Ideally, there's no distinguishing between Cleat -+- Pedal.
What if the cleat base was molded into the shoe sole? Is it now suddenly more stable? Essentially, no.
Totally different scenario and *not* relevant to stability. Already discussed this earlier related to hotspots.
A larger foot on a tightrope does not give the person more balance.
You tell us?
The interesting comment is this one.
I disagree. You could have a 1foot wide "cleat" that attaches to the speedplay lollipop and it will still be the very narrow contact width that will allow rocking in no time.
SP is a special case and it's difficult to highlight the crux of the issue due of the particular nature of the system's design. Better not to confuse any issues people have with SP with other pedal systems.
Bottom line is, as mentioned earlier, when engaged the SP pedal and cleat function as one unit. The rocking sensation you feel is only because the entire unit (in particular the cleat) is not as wide as the shoe.
If you increase the diameter of the pedal, you also have to increase the size of the cleat. But maximum benefit will arise from simply increasing the size/diameter of the cleat. If you keep the cleat the same size but increase the diameter of the pedal (and thereby the shoe having no additional contact with the pedal because of limited cleat size) the system will not be any more stable.
I will post some pictures and data later.
and it will still be the very narrow contact width that will allow rocking in no time.
Exactly. The sides of the cleats don't exert pressure to the pedal.
The "pressure" is exerted from the rider/shoe to the pedal (or cleat). You must define what "pressure" is? For now we can assume Impulse (Force for a length of Time).