I also have small belly and never fades. I do feel a lot better after a fast. I have done three fasts in last year. Two of my water only fasts were ten days and one 14 days long.
But my belly returns to its normal size in three to four weeks. My BMI is within the normal range. I am now hoping to eat good fats, which I try to do every meal, but fail a few times. I am also eating at regular intervals rather than only when hungry. I am also going to take veggie enzymes and Betaine HCL.
I am also planning to eat raw vegan diet to eliminate all cooked foods. I looked at a lot of books for Ketogenic diet but they are not useful to me since majority of them have recipes with meat. I am a born vegetarian so it would will be waste when it comes to all those books. I will have to eat a lot of avocados now.....hmmmm.
If you fast your body will heal and adjust itself, but after you complete your fast you really have to be on raw plant based diet otherwise the belly will return very quickly.