General

Thank you! I will check them out.

Do you know some good doujins with fat people? I don't care if male or female!
Also if you know where to read them that would be a great help ^^"

There's Accel World, but it might not be to your taste.

I also heard that there's a good part of Yuri on Ice where the protag is overweight.

There's a few on hentaiser if you search for "chubby", "fat", "bbw" and "weight gain" in the tags, there are good ones

Not weight gain oriented, but Satoshi Kon’s “Paprika” has a wonderful fat hero that despite the rude quips from outside characters seems very comfortable in his large size.