Chochom-ibber,
Enjoying Shabbos and maintaining a healthy diet is not a contradiction. On the contrary. An individual who makes an effort to maintain healthy eating habits throughout the week and loses it on Shabbos may become really upset.
Additionally, if you gain even only 2 pounds on Shabbos, you’re busy trying to knock off those 2 pounds all week and get stuck on the same 2 pounds over and over again.