You've asked two questions here. The title, which deals with average strength of obese vs thin, and
"A thin person, even if they train for strength, can be no
stronger than a severely obese person who never exercises?"
For the sake of clarity, I'm going to first address the second question.
Here are some numbers of The Biggest Loser contestants
On average, we have
- people weighing 325 pounds
- with 50% body fat
- thus, 170 pounds of
Ok, 170 pounds of muscle isn't technically accurate. "Fat Free Mass" also includes organs, bones, but we don't usually break it down to that degree. Regardless, you can grasp the notion: a severely obese person can have a great deal of muscle on them. (This will be even more clear in a moment.)
If we deduce the average non-obese person in America -we're not even talking super thin, we're just saying non-obese (BMI 22.5)- weighs 141 pounds, with a body fat percentage of 12, then they have a fat free mass of 124 pounds.
Muscle strength is related to cross sectional area. The bigger your muscles are, the more strength you have.
A Biggest Loser type obese person can easily have as much fat free mass / muscle as a non-obese person's total weight.
So yes, an obese person can easily be stronger than a thin person, due to having more muscle.
However, they may not be able to express that strength immediately. Plenty of obese people have plenty of muscle, but if they've never squatted before, they may very well not lift as much as a thin person, initially. They might need a few weeks to adapt to a squatting motion, so they can express that strength (neuromuscular coordination). As the first day somebody does resistance training, their legs are often wobbling after a couple sets of bodyweight work.
But sure, tossing a couch around? Moving furniture? Pushing a car where you also have demonstrably more mass to lean into the car than a thin person? An obese person can definitely have an easier time with that.
In terms of an average, that really gets into a matter of perspective.
If I have a thin client who has been lifting for months, then have an obese person try to do the same workout, the obese person is going to be in for a rough day. If we're squatting, then deadlifting, then doing walking lunges, the obese person might not make it through without being dizzy. That is, if we massage the term strength to mean strength endurance, rather than pure maximal strength, we've changed expectations. Particularly because in this example, we used some exercises which are heavily dependent on lifting your own bodyweight.
If we consider strength to be relative strength i.e. load lifted relative to your bodyweight -chin-ups are a favorite example of exemplifying relative strength- then an obese person will never be stronger. Relative strength goes down as humans get heavier (or taller).
Which brings the final point- we've only been talking external load. If we're talking absolute load, if the obese person can merely sit up and down out of a chair -a box squat- then they probably can lift more than the thin person. After all, there aren't many 140 pound people who can squat 300 plus pounds, more than double their bodyweight!
That is, just moving the obese person's weight around is resistance training itself. More resistance than most thin people will ever encounter in a gym. Sure, obese people tend to be sedentary, but it's not like they're on bed rest.