Another excellent, well reseached and crafted article on Tires and Steering geometry.
http://www.bikerenews.com/Stories/Motorcycle_tires.htm
Note the section on Tire width Vs rim width, etc. Installing a "Big Fat" tire on the back? Quite possibly not a great idea if you happen to makes turns often. Read how the "contact patch" is actually smaller if one does this, Vs. larger as first intuitions might tell us.
Fat tires on the back may look "cool" to some, but it's really recommended
only for show bikes, not for bikes to be ridden on the street or track.
That big ol' fat tire will:
- Add more unsprung weight.

- Add more coefficient of drag

- Add more friction, rolling resistance

- Place an imbalance on the handling characteristics of the bike

- Lessen the contact patch on a lean in.

Why would someone want to do this?
Ride Safe (and with excellent handling)!
Bob