IIRC there was an episode of Penn and Teller's Bullshit where they showed that overweight people could still beat thin people in a foot race if said fat person actually had decent leg muscles while the skinny person basically has puny muscles, which was to at least show that being thin doesn't automatically mean you're in "good shape" and that fat people aren't as easily winded as people may think.

This is true. The only female gym teacher at my high school was chubby, and she could actually bench 50 lbs. more than the other gym teachers (both skinny males). I myself have about 20 pounds that have been loitering around my waist, but I can still do more hiking/running etc. than my rail-thin friend. The thing is, most of these people are just sort of fat and quite active, while your typical Tumlrite Fat Acceptance Move-r is a fucking tub of lard that refuses to do ANYTHING besides whine on Tumblr.