I logged..well.."firewooded" with a big dog before. Built a chest harness for him with some burlap bags and straps from my pack, had some generic rope, grabbed axe and bowsaw and went into the woods, hooked him to some small trees, and he drug them out. He loved it! I wasn't sure how much he could do and for how long, etc, but I always tuckered out hand sawing before he tuckered out dragging small trees back (and I do mean small, 6 inch or so was typical, but over 20 feet long). I bucked them up in the back yard, just fell and trim out in the woods. Now this was winter and deep snow, I tramped and busted a path, he skidded the logs back. He was always eager to do it, too, tail wagging happy dog.
That guy ate ten lbs dog chow plus scraps a day. Irish wolfhound/english sheepdog mix