When you watch a marathon on TV, the runners look tiny, especially now that they all wear big marshmallow shoes instead of the tiny running flats of yesteryear. If you see pro runners in person, they are certainly noticeably fit, but I could see some yokel thinking that D'Amato, who is probably about 5'6"-7" isn't the right size for a marathoner. Of course, anyone who knows anything about the marathon knows that Paula Radcliffe is 5'8" and had one of the most enduring WRs.
D'Amato may be a little thin skinned as I am sure she hears all the time that she can't be a pro runner because of her age, having kids, having a non-running career, etc. and then some dodo on the bus tells her she is also too big. But it is a fair comment as I am sure many girls feel discouraged from trying distance running because they are not 5'4" and 104 lbs.