Does excessive eating indicate the presence of a devil? Will Allah punish for excessive eating and obesity? Must a believer be thin?

The fundamental principle is that the Muslim should not be overcome by the desires of the stomach and private parts. Eating a lot is not forbidden as long as it does not lead to harm, for Allah Almighty says: "And eat and drink, but be not excessive." (7:31) Excessive eating is disliked or forbidden if it leads to harm, and Shaykh al-Islam Ibn Taymiyyah went so far as to prohibit it if it causes harm. If it does not reach this extent, then the servant is not held accountable for it.