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The natural first step here is to check whether EA has lower rates of overweight/obesity than the demographics from which it primarily recruits.
I can't speak much to the US, but in the European countries I've lived in overweight/obesity varies massively with socioeconomic status. My classmates at university were also mostly thin, as were all the scientists I've worked with (in several groups in several countries) over the years. And it's my reasonably strong impression that many other groups of highly-educated professionals have much lower rates of obesity than the population average.
In general, I've tended to be the most overweight person in most of my social and work circles – and I'd describe my fat level over the past 10 years as, at worst, a little chubby.
If it is the case that EA is representative of its source demographics on this dimension, that implies that it doesn't make all that much sense to focus on getting more overweight/obese people into the movement. Obviously, as with other demographic issues, we should be very concerned if we find evidence of the movement being actively unwelcoming to these people – but their rarity per se is not strong evidence of this.
(EDIT: See also Khorton's comment for similar points.)