I watch the show my 600 pound life, it is not uncommon for one of the patients to have lymphedema in their lower legs.
Why does extreme Obesity cause this? Googling, the only thing I could find is that extreme obesity compresses the lymphatic system in the groin, which prevents lymph from draining. I don’t know if that is the cause or just someone’s opinion though.
you answered your question.
most people will die of Congestive heart Failure at 350 pounds.
For anyone to reach 600 indicates extreme variance in circulatory system.
There is simply no way to pump blood through 600 pounds of fat/
The lymph system is a direct analogy of the circulatory system - liquids flow out, but have trouble getting back.
In a nutshell, that is Congestive Heart Failure
Do you have a cite for this? John Goodman is in his 60s and weighs almost 400 pounds. He has lost it before, but he tends to gain it back.
All I could find was this.
At 6 foot 2 John Goodman is a taller than average man. Not a LOT taller (I think US average is 5 foot 10 or 11), but big enough that it might make a difference. Taller people do, normally, weigh more than shorter people.
The 350 weight was cited by the SF Chronicle in conjunction with the death (Congestive Heart Failure) of a teen girl.
There was a spirited discussion of the case on these boards.
By my memory, the question was not why she died, it was that, by all reason, she should have been dead long before then.
One case still does not prove anything save the ability of some to cite the case 
A BMI of 55-60 would be a 6’0" man with a weight of 425. According to that, he loses up to 13.7 years of life expectancy.
By comparison, the life expectancy gap between smokers and non-smokers, or between high school graduates and college graduates is also about 10 years (about 73 vs 83). So on paper, a thin high school graduate or thin smoker would have as much life expectancy as a 400 pound non-smoking college graduate.
So to imply severe obesity will kill you young, isn’t that like implying smoking or having only a high school diploma will kill you young? People with these conditions are more prone to dying in their late 60s and 70s, but in general smokers or high school educated people aren’t dropping dead in their 40s. That NIH study implies life expectancy for people who weigh ~350 pounds is in their early 70s (which is about a decade below normal life expectancy).
With any of these life expectancy stats there’s another free variable that’s not covered by the statistics but is very, very relevant to making predictions about individual outcomes: age of onset.
e.g. compare somebody who’s normal to mildly overweight most of their life but who then develops morbid obesity at age 60ish vs. somebody who’s passing 400 lbs at age 20 and still going up.
Those two will probably have different trajectories. The 10 years average number is a blend of both. Said another way, the young person is (probably) going to loose far more QALY than will the old person.
Re the OP: The bottom line is that we don’t know. But if your BMI is > 59, your risk of developing lymphedema, even in the absence of other known causes of the condition, is very very high.
It’s not clear why morbid obesity is so highly associated with lymph transport failure. A number of hypotheses abound, but none are confirmed.
We just know that morbid obesity is a very significant risk factor for lymphedema.
Thanks for the clarification. All I could find online was someone saying that extreme obesity compressed the lymph system in the groin, but they didn’t have anything to back that claim up (no scientific papers, etc).