I think this argument that is made that fat guys should stop being fat before they wast their time being UL assumes that being fat is a choice. Some of us can be ripped and over 250 easy! We are big people.
I am a big fella, its well documented. I am 6'2" and 300lbs, and could take any of you in a fight (not just because i'm big, but because i'm trained and big)!
UL is about taking only what you need and heading out. If i was hiking as a professional, i would concern myself with cutting every unnecessary ounce off my body as well. However, I, Like most everyone here, hike because i love the connection with nature i get from it. I enjoy sleeping next to the rain under my tiny tarp (not as small as yours, sadly) knowing that i can stay warm under a thin quilt instead of a bulky sleeping bag.
The less i carry the more i can explore and enjoy. My right to this desire isn't negated by my extra 50lbs, in fact i need UL even more. The more out of shape a person is the less they can comfortable carry over distance, and terrain. It also can not be assumed that a measure of a person's fitness can only be seen in lbs. I have hiked with people of all fitness levels and body types, and have always been the fattest. However, there were also many times when i was the fastest, and times when i wasn't. The less fit a person is the more prepared they need to be. I prepare for my outings by shaving oz after oz so i can compensate for my fitness deficit by carrying the lightest pack possible.
Again, if i were preparing for a thru-hike or a profession in the sport of backpacking i would be more concerned that my body be trained and prepared, but for the few weekends a year i actually get to spend in the wilderness, it isn't important enough to spend my entire year preparing for. It's just for fun after all.
-Tim (Big Tim, as they call me in the gym)