Going to the BFG's will reduce your mileage whether or not they're bigger. They just do.
I have the new bfg KO 2's and I have to say, i'm not all that impressed. They flat out suck in the rain. Sand they're good. Mud they're OK, snow they're fine.
I actually can't say that's been my experience. I have just shy of 35k on my KO2s and they are one of the best all around tires I've ever owned. Mine are fine in the rain, excellent in the snow and ice and good enough for all of my other hunting duties which can take me into some rough terrain. I lost ~1 mpg, maybe a little less going from a 4-ply highway tire to a 10-ply A/T, so no complaints there either.