When you change the size of your ATV's tires, you're essentially altering the final drive ratio. Larger tires will require more torque to turn, especially when starting from a stop or accelerating, which can make your ATV feel sluggish.
Smaller tires, on the other hand, require less torque but can cause the engine to rev higher at the same speed. This happens a lot when you change from all-terrain to mud-terrain tires, as the latter have larger lugs and are, therefore, larger in radius.
Fortunately, you can adjust the clutch on your ATV to counteract the change in the final drive ratio. The clutch plays a critical role in how power is transferred from the engine to the wheels. If you've increased your tire size, the engine now has to work harder to rotate those bigger, heavier tires. This could lead to a situation where your clutch engages too quickly, causing the engine to bog down and struggle.
By adjusting the weights (or flyweights) in the primary clutch, you can change the RPM at which the clutch engages. With bigger tires, I recommend adjusting the weights so that the clutch engages at a higher RPM. This allows the engine to build up more power and torque before it starts turning the wheels. As a result, it will overcome the increased rotational inertia more easily, especially if it is a smaller engine without any turbocharging.
You should also adjust the second clutch, which controls how the clutch engages once the ATV starts moving. By adjusting the spring tension or the helix in the secondary clutch, you can change the shift characteristics of the ATV, which can help maintain optimal power delivery, even with the larger tires.
With all that said, this doesn't mean that you can put the largest possible tires on your ATV and counter that by adjusting the clutch. Every engine has its limits! Moreover, larger tires will affect other characteristics of your ATV, like handling and braking. Larger tires will be heavier as well, meaning they will negatively impact performance and fuel economy.