That’s weird because 2 people can be the same weight and one thin and one “plus sized”.
Just saying.
My mom was 7 inches taller than me, weighed more and was thinner. I have always for in plane seats easily, but if one of was closer to “not” fitting, it would be and I weighed less than her.
So I’m not sure how good a comparison that is.
But i also don’t know how big you have to be to not fit.