I have been letting my 5yr old start learning limits and portion control. He's a bit on the chunky side too...but overall just a big kid in general. So we talk about how much a serving size of pasta is, what that looks like, how much a serving of meat is and that we can eat lots of vegetable to fill in the empty spaces. And that sometimes we're not "really" hungry but thirsty or bored so getting a drink of water and playing a game or running outside is healthier than eating.
I don't "limit" his food, but we don't keep things like Juice, or sugary/fatty snacks in the house all the time, only for special occasions. I also pack his lunches so I know what and how much he is getting....some of the school lunches are really not healthy at all IMO. At her age she doesn't need full fat milk either so you can switch to 1/2% or skim/fat free. You can switch from full fat cheese to 2% (fat free doesn't taste very good imo so we use the 2% LOL)
make sure she's getting vegetables several times a day (peas, corn, and potatoes don't count as veggies) the extra fiber in veggies will slow down the digestive process so she won't "feel" hungry so soon. Instead of giving only fruit for a snack give her half a serving of fruit, and have a serving of veggies. and make sure she's getting protien with her snacks to. Like A few apple slices, some low fat cheddar cheese, and a few peices of broccoli. Keep in mind that fruits that you can buy one at a time are A LOT larger than a serving size. When buying fruits like apples or oranges buy the pre-bagged ones they are smaller, and sometimes still more than a serving size!
You might want to take a look at her activity level, make sure shes' getting outdoor playtime several times a day esp. if she's in full day school as school tends to be fairly sedentary.
Go for a family walk every evening after dinner (if you are close enough to a mall you can walk the mall and window shop when it's cold or wet) Get her mind off food and onto family games pop in a dance video and dance with her. If you have xbox they have Dance Dance Revolution which is really fun and great excercise too!
Above all don't make her feel like the "fat kid" Don't even comment on it, just make changes for the whole family...if one person is overweight in the family it's NOT that persons problem only, it is a family issue that needs to be treated like a family issue. You can be thin and unhealthy just like you can be heavy and unhealthy...weight doesn't mean health or unhealth. i hope I'm making sense here LOL.