BMI should only be paid any regard if you have an "average" amount of muscle mass, i.e NOT if you go to the gym to build muscle, but it can be useful for the majority of women and some sedentary men.
You still have some things confused.
6 reps and below with heavy weights is traditionally thought of as the strength building phase, 8-12 is for putting on muscle mass. It's entirely possible to put on mass with 6 reps, but as time under tension reduced you'd need to have very controlled negatives.
Fat is perfectly fine in all diets, even saturated fats. Avoiding fats (apart from trans) is actually kind of "old fashioned". It's actually quite hard to put on fat if all you're eating is veg, meat and fats. Insulin spikes via the ingestion of carbs is where the problem is, although carbs can be useful for gaining muscle as long as you don't go mad.
p.s. sprinters aren't "huge"! Their power to weight ratio is insane though