Switching from an all dry diet to wet food should be done slowly and can cause a little digestive upset. It also depends on the wet food you're using....if it's a lower quality brand, it could be some of the ingredients that are causing an issue. But in general, there's no biological reason that feeding wet food should produce loose stool.
Feeding a never ending buffet of dry food is why she's overweight. My first recommendation is to get her used to specific mealtimes with a measured amount of food. Once she's used to that, then start to bring in some wet food.
A cat needs 20-25 calories per pound of body weight per day to maintain their weight. The less active, the lower the number on that scale, the more active...the higher the number. For weight loss reduce the # of calories by 10-15% and adjust accordingly depending on how she's doing. You want her to lose approx 1/2 to 1 pound per month.
Good brands include Wellness, Pinnacle, Nature's Variety, Natural Balance, Holistic Select, Fromm, Weruva, Merrick, Taste of the Wild, Innova, EVO for a start....
Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...