Braking is a big deal on a dirt bike and not just for the obvious reasons.
Motocross riding requires constant repetition and motion from numerous muscles in your body especially your hands. It might not seem hard to work the clutch and brake while holding on but there's a reason why arm pump exists. Cutting back on repetitious movement, or at least, the energy used to ride goes a long way in preserving stamina and increasing lap times.
As long as the brake lever works and the bike comes to a stop, well that's generally fine for most riders. But could you be missing actual performance gains by sticking with the standard 240 mm OEM brake instead of upgrading to an oversized 270 mm or even a 280 mm system?
Many riders swear an oversized brake system increases braking power giving you more control over the bike with less exertion. Therefore, imagine getting the stopping response needed at the touch of the handle rather than, for example, a tighter pull or unpredictable grabbing, time and again over the course of a Moto. Trust in your brake system allows you to enter a corner faster with confidence allowing for brake application only when necessary. Saving just a tenth of a second in every corner adds up even over the course of one lap.
Additionally, "brake feel" is also commonly used when discussing the difference between standard and oversized rotors. The "feel" refers not only to how the bike responds when pulling the lever but also how the lever feels to the rider. Oversized brake rotors come in two forms: Floating and fixed.
Fixed Brake Rotor
The fixed brake rotor is a simple, solid one-piece rotor system like this MOTO STUFF Oversize Rotor:
Floating Rotor System
The floating rotor system uses bushings that more or less attach a mounting bracket to the brake area where the pads grab. The bushings offer a slight bit of play, hence the term floating, thus allowing for a more centered or equal grab by the caliper on the pad surface area leading to more even pad wear. The floating system also helps reduce grabbing by the brakes which prevents lock-up. Check out this Driven Floating Brake Rotor:
Whether you use a fixed or floating system, oversized brakes also distribute heat from the pads more thoroughly which not only increases the life of the pads and rotors but likely keeps the braking power intact in a race. It's unlikely you'll ever warp the rotors from overuse, however, the oversized also helps prevent that from occurring, thanks to the heat dissipation.
Oversized brake rotors generally cost more than regular size but probably make up for it in longevity and overall performance gains. Additionally, you'll need caliper brackets (most companies include the brackets with purchase) to relocate the caliper to account for the larger rotor.
Like a lot of expensive aftermarket upgrades the choice to dump traditional sized rotors for oversized versions is up to you. There's no doubt many oversized rotors offer more style over substance but in terms of racing and riding there's a reason why the pros spend so much time testing. If the "feel" of your brakes improves when adding oversized rotors than you've just completed a small piece of the overall puzzle that gets you on the podium.
However, lots of riders find the same results just by using the appropriate brake pad for the type of riding and accompanying terrain. Stay tuned...
...and now you can read Best Brake Pad Material for Dirt Bikes.