BMI of 22 or below. A rider who wants to climb a lot better will sometimes starve themselves in the preseason and get the weight way down, then keep it down during the season simply by riding a lot. I haven't heard that they do anything special, other than weighing their food, using a balanced diet, riding 20,000 miles/year, etc.
Believe me, you ride the 20,000 and you'll climb a lot better, too. It's the biggest single thing you can do. Doesn't matter how fast, just do the miles. I ride with a fellow who rode 30,000 miles last year, full time job, wife, 2 kids. Don't say it can't be done.