Of course you gain weight on Shabbos. If I eat a pound of chulent, I have gained a pound (minus any fat burned in the activity of chewing, etc.). Does the chulent disappear the moment I swallow it?
See this Rashi from Beitza 16a: ???? ?????. ???? ?? ?????? ?????? ?????? ???? ????? ????? ????? ???? ???? ??? ????
I believe this is a spiritual form of gaining weight it is referring to. When we eat something in this physical world it weighs us down spiritually. There is also much written about the spiritual pitfalls of over eating and eating fancy things. On shabbos though, this does not happen because we have an extra neshama.
You can’t gain weight on Shabbos just like you don’t gain weight by finishing food from your kid’s plate. Also certain foods are exempt from contributing to weight gain on Shabbos: anything with chocolate, the third helping, herring, and [motzae’i Shabbos] melava malka food.
You can’t gain weight on Shabbos especially these forthcoming 4 Shabbosses, being that Ramadan supersedes Shabbos, and many people out there will be fasting everyday for this month of Ramadan, hopefully not utilizing this as an excuse for an unholy war against us.
Beats me why anyone would go to the trouble to resurrect a year-old post. Having said that, however, I have read more than once that a person who is dieting can have a “free day” without negative impact on dieting goals. Everything in moderation, but you can eat what you want.