You would have to be living in an isolated cabin in Montana not to have heard of the term "enabler".  Basically an enabler is a person that allows another person,  in a relationship, to continue with destructive behaviors by covering for them, and helping them out of bad situations caused by their poor judgment. While we typically think of the terms "co-dependency" and "enabler" as terms in dealing with chemical dependencies, it can apply equally to the "fat spouse" situation

Often the non-overweight spouse in this situation engages in "non-helping" assistance that allows the overweight wife or husband to continue to gain weight?
 

Of course,  women are the most prone to engage in this sort of behavior, but men doing this sort of thing are not rare either.  The codependency cycle works like this.  Person one fears losing the love of the person two that has the bad habit.  The fact that person two can not function without person one, makes person one feel needed and whole in a perverse sort of way. 

In other words in many cases a husband or wife can in many ways be helping a spouse get bigger and bigger by not allowing that spouse to experience the results of their "fat" ways.  Here are a few examples of this kind of behavior.

Do you ever perform tasks, that your spouse has trouble doing because of their weight?

Do you buy into the excuses that your spouse makes concerning diet, (for example discussions of metabolism, genetics, medication)  when you know very well that they engage in a lot of unhealthy eating.

Do you cooperate with your spouse's excuse making for not exercising, (e.g.  bad knees, too little time, etc.)?

Do you ever buy unhealthy junk food for your spouse at their request?

Do you ever dine at restaurants that serve unhealthy food at your spouse's request?

 Perhaps you can see yourself in these questions. Maybe you can now understand how you could be "enabling" your spouse to be obese.  Keep in mind that doing these sort of behaviors is silently approving of your wife or husbands overweight habits.  This list could go on and on.  I encourage you to add any other "fat spouse" enabling activities to the forum.