Obesity doesn't explain a $800 knee MRI or $4K ambulance ride. A californian hospital billed us at $3.7 per second for a broken wrist.
Your brother, not fat people, is making healthcare expensive. This is so freaking obvious I'm starting to veer into conspiracy theories territory failing to explain this craziness: they'll start suspecting "mongolians and puerto-ricans" as factors before they'll notice this enormous elephant in the room. Every US city has a neighborhood where "doctors live". It's sickening.
Also, ask your brother what he thinks. I have a few friends in healthcare and they're very open about it (it's hard to deny if you know their lifestyles)