Weights vary greatly with squirrels based on their geographical location. Our Florida Eastern greys weigh about 400-500gr at maturity. Eastern greys in the northeast weigh significantly more, 700gr +.
I would think that there would be weight loss due to age, much the same as you see in people and domestic animals. You can try giving additional high calorie foods like avocado and coconut. Would she possibly take formula? Make sure she stays hydrated to keep her kidneys functioning properly.
She looks good to me and 11 years old is a good testament to the great care you’ve given her.