Disabled?

I sometimes see obese people parking in the disabled car park. 

They have a disabled sign on their car, but they usually don’t have wheelchairs or walking sticks.

I don’t know if they are overweight because of their disability, or if they are disabled because of their weight.Perhaps if they parked further away and walked a greater distance, it might help improve their condition.

I am curious as to what the criteria is for granting a disabled sign.