Okay, for first, depression is a real illness. It's a disturbance of cerebral chemistry and can be treated with right medication, although complete healing process (the time the nerve system takes to regain proper balance) takes a lot of time.
Like any physical illness, depressed people can not really control all their actions. One of the symptoms can very well be that they are frankly unable to eat less, or alternately can't bring themselves to do excersize, or both, and that is nothing related to "strength of will". There can be other symptoms as well, similar in this regard. For example, someone simply cannot work. Not that they wouldn't want, or that they didn't know how to do it, they just... can't. Not a question of strength of will in case of depression. It also can't be healed by strength of will. That's one definition of serious depression.
But for most people, it's simply a bad habit of eating too much and moving too little that causestheir problem. And as difficult a habit change is, most people are not hindered by psychological problems, so in their case it really is about strength of will.