Its definietly both genetics and drive. I started at 140lbs and have been up to 240lbs at 10% bf, but it was far from easy. I consider myself the most dedicated of trainers. I havent missed a scheduled workout in over 5 years. At the age when all my buddies were out getting drunk every night, I would often choose to stay home to eat and sleep. No matter what is going on, I eat every 2 1/2 hours, always. My training is crazy intense.
But even with all of this dedication, I capped out naturally years ago, and despite trying every thing I could, I couldnt gain another pound, the only way to beat this natural plateau was to turn to steroids. So even with all the desire in the world, you get to a point where your genetics will limit you