The trays had a groove for a CD. So placing it in the groove it would work because its edges would always fall into that groove correctly all the way around the tray.
This, however, wouldn’t work on a slot loaded drive since they worked by having a set of arms with rollers that grab the edge of inserted disc and another arm with a roller that pushed it the rest of the way in from the opposite edge when it’s inserted enough.
So there’s only ever 3 slim points of contact which is fine as long as it’s a circle. Yet the irregular shape here would cause it to get partially in and then pushed by the arms into the edge/internals of the drive.
Slot-loading CD drives would get jammed if you inserted anything other than a round, full-sized disc.
The launch model Wii was an exception, with parts in there specifically for handling mini-discs for GameCube compatibility. The feature was quietly removed from later models.
The tracking on the lasers for CDs is pretty crazy, since at those scales even well balanced CDs wobble like crazy. If it had to be super flat for it to work, each disc would be much too expensive. And as soon as it got dirty or warped in the sun, it wouldn't work anymore. In reality CDs are pretty rugged and can take a lot of abusive before they can't be easily read in even a cheap reader. It's amazing technology really. It's kinda crazy to think about how many holes per sec the laser can track and read for something like a blu-ray disc running at multiple times playback speed for data transfer.
It looks like it’s pretty balanced. The chin appears to be slightly further from the center than the hat to account for the extra weight of the ears. With how leverage works you don’t need much more weight to balance as long as it’s just a little further.