The way bodies burn energy varies. Different diseases and genetic conditions regulate that. For some their bodies may burn most of the energy they take in, storing very little as fat in the process. For others their bodies store almost everything.

I provided some details about my specific case here:

https://news.ycombinator.com/item?id=49942780

To sum it up: my DNA is trash when it comes to this. Everything my body could do wrong, it does.