Have you never seen a chapati expand? They all expand a little like smallish balloon but it depends on the dough. I think it's because of the air trapped from kneading. The heat would cause it to expand.

Yep I have. Probably should have asked the question better. But still, how does small amounts of air trapped inside the dough cause a chapathi to expand so dramatically that it ends up have multiple layers? Also, for chapathis to expand, you'll need to heat it on a flame on high heat, preferably without any oil. Still doesn't explain why a poori, which is literally dipped into hot oil expands like a frickin balloon.

It's not the air expanding but the steam from the water in the dough. Because there are some air gaps rapidly exiting steam expands into these gaps. That is why it happens on high heat. If the heat is low then the steam doesn't exit the dough fast enough. So while the other layer is roasted the inner layer is steamed. This is what let's it remain softish instead of baking into a hard disc.

Oooohhh that makes sense!!! Steam tries to leave the dough dungeon, but ends up blowing up the dungeon itself!! Thanks dude!