There are a lot of factors that can lead to "fat" gliders. First you need to make sure she is "fat". Have you weighed her? Healthy adult females can range from 90-140 grams. Some more, some less yet still be healthy.
Second. There are things you can do to help her either loose weight or maintain her weight.
A wheel alone, whether used or not, isn't going to be the sole deciding factor on their weight. It helps but not as much as people think.
They need lots of exersize out side the cage as well. They need to really STRETCH out their bodies. Climbing (hand over hand action), gliding, really running around, are all types of beneficial exersize.
diet, if fed properly, in the proper amounts shouldn't cause a glider to get "fat". How many "treats" are you giving her? Try to limit the treats drastically. Instead of giving candy (like yogies), use part of her daily
diet as treats. Take the corn out of her dinner and set it aside. Then feed that corn as a "treat". Don't increase the amount of food she is getting during dinner. This way, she is getting the same amount of dinner she should be getting but she will now think the corn is a treat.