Smaller spur and pinion have less drag, so when lifting of the gas the motor natural drag brake have more resistance and less "hairpin" effect.
By changing your gear sizes like that, you should also notice that the motor moves more to the back.
It affects weight distribution and how the car drives. Usually the it is best to have the motor forward than back.
With small race size tires (41-42.5mm) the 100T spur gear can rub on the track, which is far from ideal. I've be using 92-95spur gears because of this