Main reasons:
1) Less affect on gas mileage
2) It's taller than the 265/70
3) Narrower tires perform better for a lot of my uses
4) Most tires are cheaper in a 255/70 size
The influece came from CJ3Flyr. He had a thread on the size, and the info from that thread helped make the decision. Here is his thread on the size.
No idea on the weight, I tried to find it before making my purchase on the Hankooks and never could.
Before I say my mileage, let me say that my last vehicle was a modded 96 Mustang Cobra, and I developed a pretty heavy foot. I didn't keep detailed records before the lift/tires, but I got somewhere around 16-16.5. I am now getting about 17-17.5 average (corrected for change in tire size). That number is my avearage from the last seven months. My highest was around 19, lowest around 15 mpg (a lot of towing).
No problem. I have kept record of every fill up since 6/18/2010 though an app on my phone, and it will generate them quickly for me. Very interesting when you go back looking at them. The downside is that the average also includes quite a bit of time I spent during the summer towing a 18 ft. bass boat with a 150 hp motor, not sure on the weight but it killed my numbers. If I took an average strictly when unloaded, I would expect it to be about 1 mpg higher than it is. I should also note that probably 75% of my driving is highway.