If you are in the US, please let your company know that they're risking a worker's comp suit by refusing to purchase you the legroom that you need. Protecting the health of employees on the job is not optional. They may not have the same obligation if you're overweight (unless squeezing into the seat is also injuring you), but if you are incurring injuries during the execution of your job responsibilities then the company needs to do what it takes to prevent that from happening, up to and including eliminating travel from your job responsibilities.
People also need to be aware of their body type when booking on their own dime. Cattle class is fine for a couple hours if you're less than 5'10" and less than 160 lbs. I'm small enough to fly across the US in standard economy. But if you're too big to fit in a standard seat, you need to do the right thing for *your* health and comfort.