We have a few hogs who are super chunky! Usually when we notice one hedgie is gaining a lot of weight, we adjust the food and give them more exercise.
Exercise-wise: Of course encourage her to run on her wheel a lot. If she's not liking it for some reason, try a few different wheels. I have one baby hog who LOVES to run on her wheel but will ONLY run on the reaaallly big 12 inch one that she can hardly move! When I put in the 8.5inch sized wheels that are more appropriately sized for baby hedgies, she won't run on it (despite it being the perfect size for her). And I have an adult who is always trying to run on ones that I think are a little too small for her. But they're stubborn little guys and they know what they want! We recently bought a horizontal wheel ( like this: http://www.exoticnutrition.com/flsawh.html
) and every one of our hedgehogs hated it, except our one hog Debbie. So now it goes in her cage and she runs on it CONSTANTLY and won't even run on the normal wheels anymore because she prefers the saucer wheel. You learn what they like and adhere to their preferences.
Sometimes the wheel isn't enough exercise for these chunky little guys. We have a few hogs who really do nothing
in their cages except eat, sleep, and poo. Wheels don't interest them and walking around in their cage must be boring to them. So we like to make sure that every hog in our herd gets some time in their play area at least once every two days. A lot of hogs really don't do much moving in their cage and benefit from out-of-cage play time. We use an octagonal train-table with a tall wooden barrier around the edges. We line it with the plastic outdoor table cloths that they sell at the dollar store. Super affordable and you can wipe poo/pee right off it without having to throw it in the wash like with a sheet or fabric. We fill it with tubes, wheels, balls, boxes, and anything else that they can climb on/over/through/in. They're crafty and will find a way to play with almost anything! Definitely set up a play area like that if you haven't yet. (Maybe pick up a rodent playpen!) Our hogs love this play area and even the laziest hogs will run around and play with all the toys.
Food-wise, cut down on calories/carbs. I usually go into my hog room to do food & water checks about 8 times a day. If anybody is out of food or low on water, I refill it. (Just a little when it comes to food, because they do like to tip over bowls and get bedding in them). When I have a guy who looks like he's just way too overweight, I don't continue to refill it if i've already refilled it more than twice that day. You might also want to change your food mix up if you use kitty kibble. For example, if you use Blue Wilderness Kitten chow, mix it half & half with Blue Wilderness Weight Control formula and serve him that instead for a while. Or if you use Purina Kitten chow, mix it with Purina Healthy Weight. You get the picture. That way they're eating the same amount of kibble, getting the same amount of nutrition, but cutting down on carbs/calories.
Good luck with him!