According to a recent article over at Ergoweb, obesity can be a factor in workplace injuries such as MSD (musculoskeletal disorders) and RSI (repetitive stress injuries) because "the workplace is an ill-fitting environment for the obese. Standard chairs and workspaces are too small, and the mismatch can trigger musculoskeletal disorders (MSDs) and reduce productivity."
You can find the article here: Ergonomics Finds a Growing Role in Reducing the Cost of Obesity for Employers . It discusses some of the steps ergonomists are taking (workstations with adjustability features--and teaching employees how to use them properly--seem to be key). The article also has links to other reports on the topic.