Just as in Mario Kart Wii, each character gets their own specific stat boost that accounts for kart customization, based on their weight class. The units are out of 6. In general, light characters have high values of Acceleration, Handling, and Off-Road, while heavy characters have high values of Speed and Weight. Hover over the weight column to see the characters in the represented weight class.
Here are the different characters unlocked from the beginning and their weight classes:
Toad, Koopa Troops
Shy Guy, Lakitu
Wario, Metal Mario, Honey Queen