Check this video out,
http://www.videojug.com/film/how-to-care-for-corn-snakes
in 3:30 more or less they show an example of corn snake in obesity. Itmight be that. Maybe you are over feeding it?
I also found this,
http://www.aboutsnakes.com/corn_snakes/cornsnakecare.html
Feeding your corn snake is very easy indeed and is fully covered in our free snakes ebook but young hatchling corn snakes are fed twice a week on pinkies.
As they grow, slowly increase the number of pinkies until they'll happily take 2 or 3 at a feeding.
Then move onto just one fuzzy at a time and as they grow again, increase the food to 2 fuzzies per feed.
If you've spent time watching your snake as I have suggested, you'll learn to be able to spot feeding behaviour. You'll be able to spot a snake that is hungry and is actively searching the cage for food when none is present, so consider increasing the volume diven in your corn snake care regime.
On the other hand, a snake that takes little interest in it's food and waits for more than 5-10 minutes when you provide food before digging in really isn't interested and so you should look at reducing the amount of food provided to prevent obesity.
Then when your corn snake is happily taking 2 fuzzies it's safe to move up to an adult mouse, once a week.
Beware you're not providing food that too large to be safely consumed and digested, following the guidelines in the free ebook.
I guess they are just prone to obesity.
But please, if this is not it, please let us know.
Goodluck!