Obesity is the number one most costly preventable health issue in the US. It costs companies billions more in health insurance premiums because people who are obese are so much more likely to have diabetes, heart problems, need joint replacement, etc. Statistically, obese people miss more days of work, thereby costing their employers productivity dollars and requiring other employees to take on their duties.
All that said, I don't think it's fair to discriminate based on obesity. I agree with the PP that said this is a slippery slope. Instead, I believe that employer-sponsored wellness programs are the way to go - stocking healthier food in vending machines, providing free water and gifting or encouraging employees to use refillable bottles, sponsoring "Biggest Loser" style competitions, rewarding employees who track their healthy eating and exercise, these are all reasonable ways to combat obesity that don't involve discrimination in the hiring process.