I think the evidence is compelling:
* He has lived for how long????
* To get around to every child in the world, he MUST time travel
* To get all those presents into one sack, it HAS to be "bigger on the inside"
On to the reindeer - if legend is to be believed, they are the same reindeer down through the ages. I have given this some serious thought, and my conclusion is that they use the same natural laws as Tinkerbelle. They exist because they are believed in.
Which is great - in addition to their primary function as a guidance system, they also act as a power reservoir - a battery if you will (I have ALWAYS wanted to say "if you will"!). They collect the power of the dreams and wishes of those who believe in them (children mainly, but not exclusively) through thought receptors in their antlers.
Admittedly, this is all conjecture on my part. Though I prefer to call it theory, developed through thought experiments. Much like Einstein did really.
Don't stop believing!