Syrian hamsters can grow quite large, if it is just overweight, make certain that her excercise wheel is more than big enough for her- many excercise wheels sold for hamsters are far too small (i have had to upgrade my hamsters wheels 3 times, they are now using guinea pig sized ones), as if the hamster is uncomfortable in it, it will refuse to excercise.
Syrian hamsters regulate their diet according to how much energy they burn, so it is important to make sure the hamster has a suitable sized excercise wheel and interesting cage to live in. You should also feed them veg every now and then (nothing with high sugar content in it), a couple of times a week is good- always make sure they have a full bowl of food.
You can also add a vitamin/mineral supliment to their water which can help a great deal in helping young hamsters develop healthy bones and coat, and help make sure older hamsters stay healthy and strong.
Try and let her have a run around every day for at least 10mins+, syrian hamsters can sometimes get fustrated being handled too much (although you should still make sure you handle the hamster regually none the less to keep it tame), so sometimes it is best just to take them out when they are awake (don't wake up hamsters to handle them as it can make them very grumpy, if you have to wake the hamster up, put some food by its house and let it come out in its own time) and let them run around a room while you keep an eye on their activities
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Its best to have their cage in a quiet, cool room- they sleep a lot during the day time, and can suffer badly in the heat. It probably won't be a problem right now, but check your hamsters teeth to make sure they are not getting too long- always provide something for the hamster to chew on so its teeth don't get too long so it can't close its mouth.