Apparently, some obese people have bacteria that increase obesity,
A fascinating area of study
I agree with all of those statements. People with abnormal body weights should not be used as poop donors. But I do not believe that studies support persistent or permanent weight gain. If a study has found this... I would like to see it.
You may get this study some day, but for now, the use of fecal implants in mainstream medicine is only 3 or 4 years old in the US. It was accepted because it cures C Diff - created by the overuse of antibiotics - and there was no alternative.
It is done in other countries regularly. I watched such a procedure in Europe 3 years ago.
At the clinic where I was an observer, they told me of a patient with MS, who came in a wheel chair and after treatment, something like 4 weeks later could begin walking.
This is likely due to the production of neurotransmitters by certain intestinal bacteria.
It will be a fertile field of study over the decades. You will likely see many studies.
Today, you will have to settle for healing - and steer clear of obese donors until we know more.