There are various machine factors at work here (time, speed, pressure, pad type & material, etc), in addition to other factors such as the aggressiveness of polish, environmental factors such as heat & humidity, material being worked, etc, but lets focus on the machine and pad size for a second.
Smaller pads have less surface area working against the paint / finish, which means there's more working pressure per square inch with a smaller pad. A 6" pad has more than twice the area of a 4" pad and this factor is especially important on dual action machines, hence the reason smaller diameter pads can offer more correction. A 4" pad has an area of about 12.57 in^2 where as a 6" pad has an area of about 28.27 in^2. An 8" pad has a whopping area of 201.06 in^2, or about 7 times that of a 6" pad and nearly 16 times that of a 4" pad!
When it comes to rotary machines, you’ve got another factor at work, which is the rotational speed of the pad. Even though you have a fixed machine speed of 1,500 RPM, the speed of the pad increases as its diameter gets larger. For instance, with your rotary machine set to 1,500 RPM, the outer edge of an 8� pad will be spinning at about 35.7 MPH or twice the speed of 4�. At 1,500 RPM, the outer edge of the 6� pad will be spinning at 26.7 MPH (~25% less than an 8� pad). Higher speeds increase friction, which in turn increases heat. While the smaller diameter pads increase the working pressure per square inch, you may need to increase the machine RPM to make up for frictional heat difference of their slower speeds.
What all this really boils down to is there’s no perfect universal rule that works for all products, machines, environments, etc, and you’ve got to use common sense, test methods and experimentation to find what works best for your specific situation. For instance, if you were using a rotary with an 8� pad at 1,200 RPM with good results, then you should probably increase the speed of the machine when using a smaller pad, but always work a test spot first to determine the process that works best for your given application. Even though the outer edge of an 8� pad will have a speed that’s twice that of a 4� pad, it doesn’t mean you should run the 4� pad at twice the machine RPM to get the same pad speed. You can’t forget that your 4� pad by its very nature of being smaller has already increased the working pressure per square inch and this has to be factored in with pad speed.
