I believe that giving preferential treatment to job applicants who are physically healthy over those who are overweight or obese is not unfounded discrimination. As a business owner, you are constantly seeking the most qualified person for your open positions. All other credentials being equal, it's logical to avoid hiring the more overweight applicant.
Overweight and obese employees take an average of nine sick days per year, compared with an average of only five for their healthy counterparts. Even while at work, overweight employees are less productive. Overweight female employees lose an average of 6.3 days of work production each year, while overweight men lose 2.3 days. With obese individuals the days of lost work jump to 22.7 for women and 21.9 for men, meaning you could give your healthy employees roughly three weeks of additional paid vacation time and they would still be as productive as their obese counterparts.
As most respectable full time positions offer medical insurance as part of their benefits package, with roughly 75% the cost of insurance being picked up by you as the employer. Obese males incur between $1,143 and $6,087 a year on average in higher medical expenses than healthy employees, stratified depending on the severity of their obesity. Obese females ranged from $2,524 to $6,694 in additional medical expenses. Even though you may not directly be paying these additional medical bills, you will eventually see your health insurance costs increase over time to reflect your higher risk pool.
As an employer your goal is to find and recruit the more productive applicants and to keep your overall costs down. As such, a logical heuristic would be to avoid hiring overweight and obese persons unless they are markedly more qualified for the position than their healthier competitors, enough so to justify the offsets in medical expenses and production losses.