That's because fasting in Ramadan isn't like a diet fast. You can eat but only between certain times. So if you're only eating big fatty meals late night, then sleep, that's how they gain weight.
-The fast (sawm) begins at dawn and ends at sunset.
-Each day before dawn, many Muslims observe a pre-fast meal called suhoor.
-In the evening, dates are usually the first food to break the fast.Social gatherings, many times in a buffet style, are frequent at iftar. Traditional dishes are often highlighted, including traditional desserts, and particularly those made only during Ramadan. Water is usually the beverage of choice, but juice and milk are also often available, as are soft drinks and caffeinated beverages.