Different Characters: Size Matters
Small characters are light and quick
They are great when you are way out front or know short-cuts. They are awful in crowds. Lower top speed, but quick acceleration.
Medium characters are ... medium
Faster than some, slower than others; able to push small karts out of the way, but still get pushed around by the heavy karts.
Large characters are powerful and fast
They have high top-speeds, but take forever to get up there. They can knock everyone around, but if they go off road they are sunk.