Of course the scientific way would be because of the over calorie not being burnt thus the calories stored itself into fat.
But..Why? Why can't we just eat and eat and that no harm will happen to us? When you think of it, not just the eating habit. It also applies to other activities such as exercise, studying, praying, fasting, and others, if doing it too much will become harmful eventually. The key is, everything should be in MODERATION.
To my theory it is because,
Being fat is a way of a warning that you are not doing in moderation, you are not doing the actual purpose of life. You are not hungry but you just munch away according to your taste buds satisfaction. You should sacrifice your appetite and let your pleasure of eating be treated when you have reached the eternal life. Save your money to help others, give that extra money to those in need. Imagine if eating has no limit. We wouldn't want to do anything else!Eating is a push factor for us humans to have taste of what's heading in Jannah. Let's wait for that "eating without guilt" moment.
Someone who just finished a chocolate custard tart while writing this.