Hi Noramuse,

I am somewhat in the same situation as you. I have 2 elderly female cats (14 and 13 y.o.) that I adopted from the SPCA when they were each about 4 to 5 months old. They are now starting to loose muscle mass, etc. that older animals tend to do. So I have to feed them well, and their food is down all the time. I also have 2 younger males, both 5 y.o. that were adopted through a street cat rescue organization. Jet is a slim black cat but Jasper on the other hand is a rolly polly guy weighing in at about 20 lb. He is a large boned cat, but still is round. When I got the latter 2 cats, Jasper was eating like he didn't know where his next meal was coming from. This eventually slowed down, after a few months, and he now eats less than Jet. But it is hard for me to put him on a diet because of the other cats. And...he happens to like the senior cats' senior cat food.
So he ate a lot when he first came to me (enough to make him round ), but it has slowed down.
Hope this happens for your kitty too, once she realizes that there is a regular supply of food.
Another idea...how about giving her more excercise...like chasing a pen light or one of those feather-at-the-end-of-a-stick toys?