I agree with "you guys," I saw her win a low key 800 earlier this season in like 2:05 and when she seemed like a pretty normal sized person.
Erin has actually talked about the weight thing with the New York Times:
http://www.nytimes.com/2008/08/20/sports/olympics/20bodies.html
"Donohue has her own thoughts on the matter. She says she knows she is large for a runner, something she cannot help noticing when she stands next to someone like her teammate Shalane Flanagan, who won bronze in the 10,000 on Friday.
“My shoulders are twice as wide as Shalane’s,” Donohue said of Flanagan, who is 5-5 and 104 pounds. “There is no way I would ever look like that.”
But in the 1,500, she said, speed and strength are important, so she can use the extra muscle that comes with her weight to push her way through a race. She has been running since she was very young, she said, and has developed great endurance. She runs 80 to 90 miles a week and has never had a serious injury.
Winning the 1,500 usually comes down to which runner can sprint the fastest at the end. “I have the endurance to get to that point,” Donohue said, “especially against smaller girls who may not have the strength.”
That pretty much settles the matter for me.