One of the side effects of eating disorders is a rapid sense of satiety; that is, you feel more full when eating less food. Some of this is strictly physical, as the stomach shrinks during starvation, and food empties into the digestive tract slower. The formal medical name for this is delayed gastric emptying or gastroparesis. It almost always reverses upon normalization of eating patterns, but it can take a while. Some of this satiety may be anxiety-based (that stomach-churning feeling really isn't condusive to eating lasagna, let alone with gusto), and sometimes, it was a matter of my believing my own BS. If I just ate X, I damn well better be full!
Then, of course, we have to add neurochemicals to the mix. I've blogged several times on leptin in eating disorders ( What's Leptin Got to Do With It?, Leptin and You, and Food and Sex and Leptin- Oh My! ), and the protein, which signals satiety, has been much researched in relation to both anorexia and obesity. Gina Kolata's book Rethinking Thin has a great chapter on the discovery of leptin and some of the interesting research going on about the protein.
Ghrelin is kind of the ugly stepsister to leptin--it has the opposite function (ghrelin signals hunger as opposed to satiety), and it's gotten the shorter end of the stick in terms of research (at least recently). Unlike leptin, which is primarily synthesized by adipose tissue, ghrelin is synthesized in the stomach upon eating. A drop in ghrelin levels typically happens after eating and is thought to be linked to the release of insulin, which tells the body's cells to absorb the glucose from the meal.
The increased insulin sensitivity makes sense from a biological standpoint: the body will want to make the most use out of limited energy intake, so cells would do well to become more sensitive to insulin to slurp up more glucose. Whether the drop in ghrelin is an abnormality completely separate from the insulin sensitivity, or just a really ironic side effect, has yet to be seen. However, it adds another piece to the puzzle of why people with AN find it so difficult to eat more: they really do feel full.