How to get fat while keeping the fast:
Try to subsist on pasta.
Many people do this, and “can’t understand” how they can gain weight while keeping a strict fast.
If you try to live on pasta, with various sauces on top, you’ll always be hungry, and unless you have more discipline than almost everyone, you’ll end up eating a lot of pasta. And you will get fat.
Yes, go ahead and make use of pasta, but make sure you’re also eating beans (and peas & lentils), whole grains, green and yellow vegetables, and so on. Eating less will seem much more imaginable.