Mark is built just like Miles when we meet him because he has to look exactly like Miles. He is starved to get that way. Actually, I don't know if the musculature is the same--I assume they look identical with clothes on (meaning same size and shape), and presumably have the same scars, but they might actually look subtly different without clothes. I dunno. I think Mark chooses to become *obese* (I know, chooses, obese, loaded words) later on, but if he eats enough that he's not being starved, he will be quite a bit heavier than Miles. My mental picture of him as "built like a tank" is from near the end of Mirror Dance, when he was both fat and strong. I don't know if Miles is as strong physically as Mark. I think he probably couldn't be, because if he exercised his muscles that much, they'd put too much stress on the fragile bones. (My sister has our dad's ability to develop lots of muscle combined with our mother's bird bones. It's caused her problems with things like her neck going out, and that's with normal-strength bones that are just very slender. I can only imagine that Miles would have worse problems.)