It's not the weight itself causing diabetes, and it's not the calorie reduction that fixes it.
Weight is a correlated with diabetes. Too much insulin causes both.
The calorie reduction works because if you cut out half the amount of food you are cutting out half the stuff that's causing a problem. But you don't have to cut half the calories. You can just cut out the stuff that's problematic and eat more of the stuff that isn't.
You get healthy to lose weight, you don’t lose weight to get healthy.
Weight loss is a by-product of healthy choices.