Why we have a greater appetite on Shabbos:
On Shabbos, the extra soul which a person merits causes him to have a much greater appetite than he normally has during the week. This extra soul is a revelation of G-d within the person and causes him to desire to eat much more than usual. On this day, Hashem receives tremendous pleasure from the enjoyment one receives from eating food.
 Or Torah [Maggid] 28