Rob is right, but maybe not clear. A smaller front or larger rear will increase the amount of torque delivered to the rear wheel. This will increase acceleration in any gear, while hurting the overall top speed. The inverse is true. A larger front or smaller rear will increase the potential for speed, but acceleration will suffer. I have experienced an occasion where a larger rear and smaller front have resulted in a higher top speed (from 99 to over 108mph) on a street bike. This was because the engine power could not overcome the drag in 5th gear. This is not typically the case on a dirt bike as my KX250 makes more power than an 800cc cruiser.