Most of these elite athletes are also very fine boned ectomorphs. I could be 10#s underweight and still have cankles. I just don't have delicate bones--I'm OK with that (well, except for the fact that I will never have shapely legs, since my legs are so tree-like)...I'm not at risk of breaking them, either.

The ideal weight range for my height is something like 115-135, but when I get much below 125 I start looking...awkward. I have big boobs (had 'em reduced and am still a D), and a big butt and thighs. When I get much below 125 my collarbone and wrist bones start looking a bit to...bony. I'm a pretty solid mesomorph. Were I to have that ectomorphic body type of an elite endurance athlete I'd wager that I could be 115 or a bit less and not look too thin for my build.