What they're doing is they're eating more than the normal mouse, so they only get fat if we let them  eat as much as they want.
والذي يحدث انهم يأكلون اكثر من الفار الطبيعي وبالتالي هم يصبحون سمينين اذا سمحنا لهم بالاكل بالقدر الذي يريدون.