Narrow Tire and Braking
It probably does reduce total amount of braking force, but things like weight distribution, tire construction and rubber compound probably are more important factors. I didn't think the front brakes on the 900 Custom felt all that different than the front brakes on the 900 Classic, or any other mid-sized cruiser I've ridden.
The rear-heavy weight distribution limits braking effectiveness of the front tire, so even when there's a 17-inch radial and dual four-piston calipers with floating 310mm rotors on a cruiser it still doesn't feel like a sportbike's brakes, even if it does actually brake harder than a sportbike because of increased effectiveness of the back brake.
-Gabe