To be blunt, if she's gaining weight and does not have any underlying health issues, then she's eating too much for her activity level. If you want to keep the amount of food the same, she needs more physical activity to be healthy, no ifs, ands, or buts about it.
My indoor only cat is 11lbs, a neutered male, and 3yo. He is at an ideal weight for his age and build and has no dangling belly flap. He is fed twice a day - each meal is 1/4 to 1/3 cup of dry food. He doesn't get a ton of exercise (and is restricted to one room due to inappropriate elimination issues), but he gets food from a toy occasionally, we have toys hanging from his cat tree, he has two scratching posts, and a couple shelves to climb on. If he starts to gain weight we reduce his food a bit, if he needs to gain some we bump up the ration a bit, just like with our dogs.
For interactive activities, we use a few teaser toys, a laser pointer, and Pi likes to fetch at times so we'll also throw his stuffies for him to chase if he's in the mood.
Letting your cat outside, especially when you live in a heavily populated area like an apartment complex, is *not* an option - it's dangerous for your cat and annoying for your neighbors.
I'm not a huge fan of their foods, but Purina has a well-illustrated body condition guide for cats that you may find helpful:
http://www.purina.com/cat/weight-control/bodycondition.aspx
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