In Mario Kart Wii, you can play as your own made Mii.
It sounds nice, but, see it like that:
You have a big and heavy Mii and it has a stupid voice.
This will make you have to choose heavy weight cars.
What to do?
Erase your data?
No, see it yourself.

Here is your Mii.
Let's see what cars/cycles it has.

No!
I have heavy weight karts!

No problem!
Just go back to the Wii Menu.

Go to the Mii Channel.

Edit your save file's Mii.

Before:
Big.
Heavy.
The voice you don't like.

After:
Small.
Light.
The voice you might like better.

Start up Mario Kart Wii again.

Here is your new Mii!
Let's see what cars/cycles it has.

Yes!
I have light weight karts!

As you can see, MKW has the freedom to change your Mii anytime you like. :)
I personally like the settings done in this tutorial better, since the small male Mii has a fucking annoying voice.