Why we have a greater appetite on Shabbos:[1]
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.
_____________________________________________
[1] Or Torah [Maggid] 28