As has been pointed out, how much one eats or does not eat--and one's weight gain or loss--is also a matter of emotions and personal control. Tatiana was naturally slender--and of course, she had tremendous discipline--but I rather think that by the time Olga arrived in Ekaterinberg she took her meals simply out of rote, and ate just enough to keep her alive.
Exercise is naturally another factor. We all know that exercise is a great way to alleviate stress, and so Nicholas is not to be faulted for his focus on taking exercise while in captivity. The girls also enjoyed exercise, and you've probably seen photos of Nicholas and Alexei sawing wood, and Olga chopping wood. As for Alexandra's lack of exercise, due to her various ailments--well, that's undoubtedly just one more factor adding to her ailments . . . one chasing the other.
And by the way, one of the more moving things I have read about their final days is that the cook, Kharitonov, was in the process of teaching the girls how to bake bread.
