It's also possible that her limbs and muscles are weak for whatever reason and she is unable to move at 600 pounds because of that. Height certainly plays a role, but it's not the only thing to consider- the body's ability to carry that weight isn't going to be constant from person to person.
And if she gained a lot of weight due to an injury (a friend of mine on another board suffered two leg injuries and gained a significant amount), that might also be part of why she has difficulty walking. My friend was on the verge of immobility at just over 500, if I recall her story correctly; and even though I know of many others who're highly mobile at that weight and higher, such was not the case with her simply because of the reason she got as large as she did in the first place.
13 years