I think a TLR is the best performance/weight tradeoff for backcountry photography, on the whole. Large format is more of a "work out of the car" proposition, unless you have a really compact kit; I've carried a whole mess of plate-camera stuff miles into the desert on a hot summer's day, and while it *worked*, I have to say it wasn't really worth the blood, sweat, and tears as compared to medium format.

For me, water is the big contributor to weight and limits the length of a trip: one to two gallons per day, at eight pounds to the gallon, and it outweighs the camera gear mighty quickly. If you have access to drinkable water in the field, that makes an enormous difference in packing, and the pack space I devote to water would probably allow you to take a 16x20! :-)