Why do I lose weight every time I eat some junk or fatty foods?

I've a problem! Every time I eat something really fatty like pizza or Amy type of junk food, I lose weight instead of gain? I eat super healthy but in a blue moon I have a cheat meal. After my cheat meal, I notice that my clothes gets bigger & that I lose weight. Why is that?
Why do I lose weight every time I eat some junk or fatty foods?
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