So these were found out not too long ago. After much analyzing, some people figured out all of the weight classes in MK8, thanks to a new demo available in Australia.
People thought there were individual characters stats at first but it turns out Nintendo just led us on to believe there were less weight classes than initially thought, as usual.
The seven weight classes are as follow, from top to bottom:
- Feather (Baby Mario, Baby Luigi, Baby Peach, Baby Daisy, Baby Rosalina, Lemmy)
- Super Light (Toad, Toadette, Koopa, Shy Guy, Lakitu, Wendy, Larry)
- Light (Peach, Daisy, Yoshi)
- Medium (Mario, Luigi, Iggy, Ludwig)
- Cruiser (DK, Rosalina, Waluigi, Roy)
- Heavy (Metal Mario, Pink Gold Peach)
- Super Heavy / SHeavy (Wario, Bowser, Morton)

*Note: Mii's stats are unconfirmed and are just speculation. Everything else is accurate.
That's it. For TTs, you can bet Wario/Bowser/Morton will be the 3 go-to guys.